Also have you considered putting a AVR on the shield so the code required to interface with it is reduced?
Put all the low level code on the on board processor and expose a simple API over I2C or something.
I agree that would be pretty cool and not only for arduino but avr/microcontrollers in general. Just make sure that we can still program the onboard avr as well .... Darn, now I wish I thought of such a shield with the onboard avr idea. Let me know if you need help with the firmware for your onboard avr. if your planning on doing it ... 