My recollection is that the original NES cartridge used two ROMs, one as a character generator and one with the program code. What I did with the SuperCart was create a complete Z-80 computer with its own ROM and RAM, and then mapped a second access stream into the Z-80 RAM for the NES console.

The Z-80 could then modify the character generator and the NES's program code in real time. For some applications (especially player's-eye view or "3-D" rendering) we would actually replace the entire character generator during horizontal retrace, so that we had pixel-by-pixel control of the screen -- something you couldn't do with straight NES cartridges even if you had the processor power (which you didn't). So the effect was that we greatly increased the effective amount of available ROM, we improved graphics control, and we had a complete Z-80 that could run as a graphics co-processor.

I only know of two games ever written for the SC: Koala Chase, which I wrote mainly for testing the SC and never finished (I don't think it ever saw the light of day), and one commercial title that CD produced but I never saw. Can't remember what it was called.