Thanks for taking a look Sceadwian. I did some tests with this using some Radioshack grade PNP BJT transistors, specifically the PN2907.
When the 5v base is left unhooked/floating, which it will be when USB is not connected; the ransistor does not pass any voltage as far as I can tell.
If I tie the base to the 7.2 - 12v ground, then when I re-hookup the 5v line, the 5v+ will not be enough to overcome the ground potential of the 7.2 - 12v ground, so the transistor passes voltage no matter what.
I think im going to take a different approach to this. Thanks for taking a look.