Weird-ish question here.

I've got a partial idea swirling around, and it wants to play with a specific scene. But I'm having a bit of trouble with part of the setup.

The scene in question involves a changeling requesting assistance from Rarity (she needs a fancy dress for a special event). The catch is in that the following are true:

1) The requested dress must be designed for the changeling's natural form,
2) This story idea takes place not merely before the wedding, but before the show begins, and
3) As of the wedding, Rarity didn't know that changelings existed, let alone what they were.

The above wouldn't, normally, be a problem - just have the changeling brainwash Rarity into blind submission, right? Well, this particular changeling isn't one of Chryssi's brood - she's from a different nest, with a sizable pony population to it. And because of this Snow, the changeling in question, has been raised with the belief that it is unimaginably rude, and socially unacceptable, to just brainwash a pony without asking permission/seeking consent.

This ties back to the problem - given that Rarity didn't know changelings even existed, is there any way that Snow could word a 'consent to brainwash' request that would fit the following statements?

1) Must be something Rarity can freely, should she choose, agree to or disagree with.
2) Must be something where, if Rarity does disagree, Snow can walk away without brainwashing Rarity or revealing the existence of changelings to her.
3) Must be something with at least a semi-decent chance that Rarity would, in fact, agree.

Any thoughts on this?

Well, perhaps a very detailed NDA (non-disclosure agreement) where Rarity acknowledges that, at the end of the job, she consents to having her memories of the identity and appearance of the client modified. Also to destroy any patterns and measurements and to anonimise her accounting record.
She signed the contract believing her client wound be some royal or something.

6817514 It wouldn't even need to be a very detailed agreement, or remove her memory of the client's identity - just of the client's physical form. The changeling could hint that it's very sensitive to its privacy, and perhaps its cutie mark.

Why would a changeling who lives in a society where changelings and ponies openly live together peacefully want to brainwash Rarity into forgetting that they exist? It seems weird.

I don't personally see a person agreeing to have their mind wiped ahead of time as part of a contract. Such a thing is incredibly suspicious and would likely warrant immediate refusal and possibly even notification of law enforcement, unless a very large sum of money were offered, but even that could just add to the suspicious nature of the request and could backfire if done with somebody with scruples. I think the reason you may be having difficulty with this concept is because it is just simply crazy. Unless the consensual brainwashing (or brainwashing in general) is integral to the story somehow, or the tone of the story is such that something crazy that doesn't make sense fits (as in definitely not a serious tone), I'd probably ditch it.

Why not just have the changeling transform into a black pegasus with the same dimensions as in their natural form? Same colour scheme means that the colour choices won't be affected with the dress. I suppose the accommodations for the wings won't be quite right, but be a picky enough customer and it would probably still work. Dresses don't tend to go on horns, and it would be a truly odd (though perhaps not impossible) choice to have the dress going inside the holes in a changeling's limbs.

Alternately, provide the measurements via mail for her to make a dress, or provide an unusual mannequin for it in the shape of the changeling in question. Heck, they could say it's for a weird, life sized doll rather than for an actual, living individual.

So given the care for consent, no usual fey word traps...

I would actually have Snow mostly be upfront with her, but limited. Approach as a pony and say that she is the representative of an unknown species in Equestria, and that 'her client' greatly values her privacy but nonetheless has learned of the extraordinary quality of Rarity's work. She is willing to pay handsomely if Rarity agrees to have a legally authorized, witnessed, third party contract unicorn take custody of the memories of the actual design itself.

If that's too straightforward, have her pose as eccentric and suggest to Rarity that they can make such a fabulous splash that EVERYONE will be dazzled by the work...even the designer, who won't remember that she made it. "I wish you could have the joy of seeing a creation as fine as yours unveiled for the first time. Although actually, there is a way..."

Thinking about it a little more, I can come up with a way for this to happen in a more serious manner.

The biggest issue I have is "why would Rarity ever consent to having her memories wiped?".

You say this occurs before the show begins. If I were set on making this work in a serious setting, I would have this take place shortly after Rarity first opens up her business. She's not very well known, she doesn't have a lot of loyal customers yet which means her general income is low, and she has debts to pay from the loans she needed to open her business. She is very conscious of her financial situation and near desperate to get some good paying business. Under those circumstances, I could see her consenting to a memory wipe. It's not a clean consent, since it's based out of desperation, but it is consent. Have them appeal to her vanity (I can't believe how underappreciated you are! You are marvellous at your job), appeal to her creative curiosity (I have a very unusual client, why, she has a body type you've likely never even seen before, let alone designed a dress for. It could be quite the adventure for one as talented as yourself.) and of course, appeal to her financial need (I know you're hard up...we'll pay generous rates, if you agree to some conditions...).

It doesn't necessarily need to be a firm yes/no to the very specific act of mind wiping/brainwashing. It could be a simple as a question of whether or not Rarity would be willing to keep a secret. I mean, as far as a changeling with mind-wiping powers goes, that's the consent right there. Is it a little devious, yeah, but guess what, so are changelings. So is mind-fuckery for that matter! With or without consent. From whatever headcanon 'brood' the changeling is from, really doesn't matter. They are in one way or another, parasites. And they exist because they are devious.

It's not outside the realm of belief that these changelings are less malicious and believe it's immoral to 'steal' love without consent. But that doesn't change anything. What I would find hard to believe is that a bunch of changelings go around asking ponies if they can mind-wipe them. So, in my opinion, keep it simple.

"You see, it's a little complicated. So I have to ask you: are you willing to keep me and my dress and everything about this transaction a complete secret?" she asked.

well for one rarity takes her profesionalism very seriously, her reputation is dependent on customer satsifacton and trust after all but honestly even now the simpelest way to prevent a problem would be to have her make a pinkie promise...or rather to have snow do so.

"um before we begin there are certain aspects of myself that cannot become common knowledge and while i do trust that you are a profesional and would never willingly devulge such things the importance of this is paramount to me so i must ask of you a form of binding magical contract in this, you will remember as a result only what i wish you to of our encounter today and henseforth if you agree to it, and on my end i agree to not abuse this in any way shape or form, cross my heart hope to fly stick a cupcake in my eye" she would say followed by the standered 'ouch' because she forgot to close her eye for the hooff to eye gesture just for the ususal comiedy moment

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I say that removing the identity of the client would be important enough for a well-rounded magically-enforced NDA. Standard procedure, even.
Afer all, you cannot reveal what you do not know any longer.