No, I want to keep this one simple and all via the address bar. However, I wouldn't mind making it more generic and for PMing friends, all offline users, certain groups, etc. I think what you guys are looking for is
Zero Tolerance's Mass Admin PM, maybe talk to him about porting it (or get his permission for someone else to).
I may take that suggestion and make it more generic, but before I do that I will be porting this for 3.0.x (for the guy who originally made the request, I made a boo-boo and made it for 3.5 :ermm: )