Why not simply have Plus! say when someone opens a conversation window "I'm using such and such, what about you?" (ok, not in actual text, but you know what I mean
)
No response -> No Plus, thus you could make messages look like,
Blah Says:
Blah has just tried to send you an Plus! Sound, but you don't have Plus installed, etc.
A response, wrong version -> Upgrade Message
Correct Response -> Etc.
Wouldn't be to hard to do. I think the biggest problem would be making it so that you have to have checks around every feature to check the reciever has the right version.