I see where you're coming from. Doubtful they'd let you PTAD out of Mike Co, though stranger things have happened. You'd be more likely to get PTAD orders cut before you reported to TBS, but that would defeat the purpose and only delay the inevitable.
If you're not picked up for the company you have orders for, I'd say your best bet would be to take leave. I'm not sure how Mike Co handles leave, but I've heard all sorts of scuttlebutt as to it being limited in certain ways (not sure, as I haven't spent time in Mike and after all, its just rumors).
Bottom line, your life would be easiest if you sold your house, got new digs here around the Quantico Highlands, do your TMO, and show up as early as possible the day you check in to ensure you got a spot. Easier said than done, right?
And remember, common sense is an uncommon virtue here at TBS.