I don’t know of a plugin that could do it, but it’s a nice idea!
A couple ways you might go about it:
1) Filter the output of bp_has_groups() to unset those groups that the user is not a mod/admin of. Pro: You could do this pretty easily right now. Con: Not terribly efficient in terms of queries and cpu cycles
2) Write your own custom MySQL query to get those groups. Pro: Efficient. Con: You don’t get to use the group loop.
3) Submit a ticket at trac.buddypress.org to have that kind of bp_has_groups() parameter put into BP core. Then you get the best of both worlds, though not until at least BP 1.3
Good luck!
