Could Flash try and tell known and trusted friends in the Guard (for example Steel Wind and Sharp Spear) in to the Order of the Silver Guard?

That's a maybe, but on a personal note, I don't think Steel and Spear would be able to keep quiet about it. Others who are more reasonable and trustworthy might be worth a go (Like Shooting Star), but of course, we don't know if he can introduce them without permission.

Too many eyes around Canterlot, and I don't mean Discord. Canterlot is the center of the whole Order, and just about anyone who would have the most to lose from it being upset (AKA, the nobility and other high ranked Unicorns) live there. Really Canterlot should be the LAST place to set up any overt resistance to it. Plus I agree with Brawny in that those two would probably be blabbermouths about it and screw the whole thing before it got started.

Thank you, Brawny and Arcane for your insights. Shooting Star is already a member of the Order of the Silver Guard. He was up in the Chrystal Empire to help Shining train the recruits. Maybe Flash could try the few unicorns in Ponyville, if he ever gets to go there.