MadRat: Sure, easy job. Write yer BIOS to map the graphics card's linear frame buffer from address 0 upward, do not enable the board's own RAM controller at all, and there you are.
You'll be surprised about how much slower the path through the chipset onto the AGP bus onto the graphics chip's RAM controller is than the short path straight to the chipset's own RAM controller.
And of course, by doing that, the graphics card would become unusable as a graphics device.
Or design the stuff to specifically do this, and put the unified RAM directly on the chipset. What'd you get then? Right, a shared-memory graphics solution. As seen on low cost chipsets since 1997 - only combined with really fast RAM. Kind of like the very latest SiS chipsets do.