Are you worried at all that people will add things that they aren't interested in - just to see if others are? Curious what you've thought about that, and how you plan to mediate it. Definitely good ways to do so. BEST OF LUCK! Identity is a majorly messed up space, and this is a much better direction.
This is why I liken overlaps more to 'tact' than 'privacy'... because you can't prevent people from lying/misrepresenting themselves, nor can you prevent people from changing honestly wanting to change their overlaps over time (you might be Christian today, but atheist tomorrow...).
One possible solution is to only let you leave an overlap a week after you've joined. But then you have an incentive to create a fake account, and you can't use it for short term overlaps (like joining "I feel like shit" for four hours).
Gotcha - makes sense. Could be fun to go into anonymous mode, or something of the sort. If you want to drive true identities just make each overlap a public declaration (you have to tweet to add) or something like that. Always tricky where to place your barrier to participation ;)
