I've got an idea playing at the edges of my mind, but it's running into a problem with a logic point. I'd like some assistance with determining how to set this up so that the problems are minimized.

The setting is some time before the events of the show. The scenario involves a changeling ('Mimic') who has obtained a noble title from Princess Celestia, and a small village as her holdings to go with that title.

Celestia knows what Mimic is, and trusts Mimic. Ordinary ponies, however, wouldn't trust changelings. (They are, at this point in time, monsters from legend.) Mimic intends to use her new town to attempt to bring changelings out into the open, hopefully with Celestia's blessing. But this has to be done carefully - otherwise, there will be panic, and things will turn out even worse than they already are.

In order to resolve this issue, Mimic intends to set up a barrier around her village - while ostensibly for the protection of the villagers (which it will, in fact, do), the barrier has a secondary effect - anypony passing through it into the town will receive a minor dose of mind control, locked to a single idea - that there are four, not three, basic pony tribes. Anypony leaving the town, on passing outside the barrier, will receive the reverse information, matching expectations in the outer world.

That is the background for the situation. My questions are these:

1) How, before the initial casting of the spell, can the purpose of it be announced in such a way as to offer the current inhabitants of the village in question a chance to leave if they so desire, without giving away the presence of changelings among them?

2) How can the spell effect be phrased in such a way as to make it something both at least marginally legal by Equestrian standards, and more importantly, something which Celestia would have little-to-no trouble accepting as a valid use of magic?

if mimic is pretending to be a unicorn, have mimic say that a somewhat dangerous experiment is being worked on, and the wall is for safety, telling ponies if they're not okay with it. It has a little bit of truth to it. Mimic is trying a dangerous experiment, of a different sort.

Tell Celestia that Mimic is trying to make changelings more accepted into the population, to help everyone by making ponies not so scared, and changelings more willing to work with them. An experiment in cohabitation.

Just what I think.

1. Mimic will make a new village, and call it a social experiment, inviting ponies to come live there, with perhaps some kind of perk or bonus from the government (Celestia's contribution). Perhaps no income tax, or subsidy for housing or something. That gives the ponies a good reason to want to be in the town in the first place. Have one of the stipulations be that the villagers are not allowed to leave the village for X years, unless they are leaving permanently. So they still have the free choice to leave whenever, they just won't be able to come back... because they might be able to figure out what's going on if they did. I'd say it would be ok for other ponies to come visit the village, this allows for trade of goods and information, so the villagers won't feel cut off from the rest of the world.

2. As it's officially a 'social experiment,' it wouldn't be a stretch in a magical world to assume that the influence of magic would be part of the experiment. I wouldn't ever use the phrase 'mind control' unless it was used by a character who's pissed off about being mind controlled. In fact, the villagers shouldn't even be aware of the purpose of the magical influence, or what it does exactly, as it would normally be with any experiment involving test subjects. Though they should be aware that there is some kind of magical influence, for purposes of transparency.

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What if he announces that the town will be accepting to all creatures who wish to live in harmony. When ponies leave, the changelings they've encountered could be substituted for buffalo, kirin, hippogriffs, griffins, and yaks. Then:
1) He says the spell is an experiment, and it will make ponies think that living among other races is normal. (which is basically true).
2) He uses the same pitch with princess Celestia.