If you really, REALLY want, you can just make an adapter for the MXM port. It's just a normal PCI-E 16x port with extra pins to output the video to the internal screen. Just make sure it's an Apple card for the Mac Pro or you're limited to just Windows.(Don't forget about that 8800gt upgrade problem.)
Edit:
Nevermind, found this:
Seems like it won´t work to change the MXM card onboard