I also think that people would like a nice solution like that. But as fas as i know there is no EFI Firmware for the GTX765M. If so, some guys (like me) could make a lot of money fixing dead iMacs. Cheapest card i have working is the K1100M, cost 30 Euros and is faster as the 6970M, and uses less power (only 40 Watt TDP, instead of the 75 of the GTX765 and 6970M). If it would be possible to get a Boot screen, Backlight control and no problems while upgrading to a newer OS, lots of companies could making lots of money.
So the question is will a K100M fit (MXM-A) / work ok on a 27" imac (MXM-B) ? Any particular brand / origin I should look for?