The only problem with the metal armor bit is that if one interprets that restriction to be one of obedience and adherence to a taboo, there's room for the player to argue that his god (or Mother Nature) would know that the armor had been forcibly placed on him--and by that arguement, he ought to lose his powers every time someone stabs him with a sword, because that's metal too. You're basically changing a spiritual vow into a physical allergy to metal.
At the very least, for internal consistency, you should have the villain use Suggestion to get the druid to put the armor on himself, rather than holding him down and strapping it on. That way, you can tell your player that his god feels he's supposed to have enough willpower to overcome such magic, and if he doesn't, well, sucks to be him for a day.
I do remember that Oriental Adventures had a few spells that dealt with preventing or reversing shapechanges...perhaps a magic "trap" with a visual trigger that activated whenever there was an animal in the room, set to cast "Force Shapechange" (or whatever the spell was called) with an automatic immediate reset. As a bonus, I believe the spell does damage when it forcibly reverts them, so it would have a nice, "Don't try it again!" implication. Also, a rogue would have a chance to disable it like any magic trap, which would give the party rogue a nice way to contribute to the escape.