Is there a chemical that only reacts with DMT and can be readily observed to have reacted with it or that can catalyse a second visible reaction? If not, what would it take to design a chemical "lock" that only DMT can unlock?

My particular use for such a thing would be to pulverise or gooify a little bit of whatever plant I find and determine any significant presence of DMT. But no doubt it could also help with purity testing.

I reckon if such an indicator was known, it would be in use already.
DMT has a lot of common chemical cousins in nature, which might make it difficult to find some kind of reagent that can reliably be used to reveal DMT specifically. But I'm no chemist. Who knows, such a chemical might exist out there somewhere.

That's pretty much why I made the TLC kits. It's a little bit more complex than what you describe but not too much.