I just can't see any counterplay.
"Hey they are using my work. I want something!"
"Well duh you did volunteer it to an open source project with a clearly defined license."
I can't see that they would have any ground. I think they would have to explicitly copyright anything they wish to establish ownership of.
It might be worth posting a question up on http://law.stackexchange.com/ just to see if anyone can see a potential dodge.
But it looks very low risk to me... Worst case scenario you get a letter from a lawyer, you would have to revert the relevant commit and rewrite the contentious code.
And in this community it would be such unlikely behaviour. It is a blatant dick move, biting the hand that feeds.
Just be careful you don't start getting tangled in red tape! It would be a pity if that got in the way of tapping into a potentially valuable resource (community dev).
I suppose you could create a "contributors agreement" that you get contributors to acknowledge by "tick the box" to automatically defer ownership of future commits to ROLI. You could mail it to them the first time they commit something, or it could even be in their profile page in the forum. "By checking this box I hereby agree that any contribution of mine may be used freely by ROLI unless I explicitly state otherwise. So long as the user account has the same backing email as their GitHub, it should be good enough.
Final idea: maybe a sticky giving guidelines for contributing: (1) tick the agreement and mail it to Jules. (2) make sure you don't have your own copyright anywhere. (3) if adding files, be sure to copy the JUCE license over.