Shameless bump
I suppose what I'm looking for is more of a Distribution List type of thing.
Member creates a named 'group' of members, and then they are able to type in that name in the To for a PM. Obviously already-taken names would be prohibited for use as a group name.
Ideally it would parse out the group name and list out all of the member names, possibly with a check box next to each to include/exclude users, if someone wanted to do a one-time edit of the list.
Even more ideally, a message would be displayed if they are attempting to send a PM to more people than they are allowed (per Usergroup).