I know that myself and many others are easily deterred from servers if they have things like 'please agree to our TOS before playing'. This is more or less what this suggestion entails.
As
LieutenantIV mentioned, it's quite easy to create workarounds in hacked clients. Sure, the developers can follow their changes and patch them, but the client developers will always be 1 step ahead. Hacks are also a part of the game, believe it or not, even though it's one of (if not the) most frowned upon aspects of the game. There will never be a hack-free Minecraft.
Sorry, the idea works in concept, but in reality I don't see it being too successful.