I believe EMC2 is x86-only, though I may be wrong. However, I'm thinking of using my rpi (if I ever get it):
run all non realtime pins to r-pi gpios (things like heating control, stepper enable lines, fan control)
use small avr connected to the serial lines to do real-time stuff
modify pronterface to poke the gpios instead of sending gcode to the avr
