I have come across the same issue exactly. Anyone got an answer to this one?
Also it might be better to be able to assign bugs by the Status so for each status for a project you can assign the bug to a particular user.
On turnovers of a build with fixes we go to Resolved and would like to have the bug actually unassign (no one) or create a dummy user xxx Turn Over and assign it to this. That way a group of testers can pick and assign from the group of bugs turned over.