To log PIC time you have to have the ultimate authority above anyone else on the airplane. Just because you are sitting in the left seat does not give you the experiance that comes along with the responsibility. That's the problem. It's also in the FARs. If the captain agrees to sit right seat, he would have to concede authority to you, highly doubtful. The exception sort of lies during upgrade, when in OE. I hear you technically can't log PIC then either, only the check airman can even when he sits right seat.
I am there with you though. I wish there was a way for FOs to log PIC. That would help us out a lot. Especially during these times of forever upgrades.