Well dpc+ and dpc optimised video and dpc+ added extra sprites (didn't pitfall 2 use dpc). Also "ball blazer" for the 7800 used the pokey chip for its awesome music. The starpath super charger enhacned the graphics and made one of the best versions of froggeron the system. Also, there is the harmony cart that gives the ability to load game from an sd card or you could download games to the cart.
But once you are running an ARM processor with megs of RAM in the cartridge and bus stuffing on every cycle, I have to ask "why?" - just to hijack a TIA?
I suppose it would be to stretch the definition of what you could accomplish from the 24 pins on the cartridge port. Meh, if you make it, the demo crowd will probably make some really impressive demos.