Bummer.  But not terribly surprising.  Is there a ROM on the card?  Or is it totally dependent on the driver loading the FPGA image.  I guess there must be a declaration ROM at least.   Might be interesting to read that out, if there is one.  It's possible that the declaration ROM just declares what model card it is and then the driver loads the FPGA code for that model.  Possibly, change one byte in the declaration ROM and it might know to change the FPGA code to support the larger VRAM.  

It'd probably make more sense to look for the test and branch instruction in the driver, than to spend a lot of time guessing at the contents of the declaration ROM.