Okay, never mind. I examined a 5500/6500 board last night and there are no traces to pin 30 of the VRAM chips, so the needed extra address line is not supported. Darn. I have like 2000 of those chips on hand. It would have been nice to use 2 of them...

On the other hand, the extra address line may be available on the Rage II chip and could be wired into place. But that's more effort than I'm willing to exert at the moment. Anyone have a pinout for the Rage II chip and/or a datasheet?