http://rowebots.com/products. If you can get that OS to multiprocess then I reckon microchip would be one option I think better than Parralax. I havent kept up with the scene but  around 7 years ago this http://geocities.ws/billy_stiltner/worldrecordplane/worldrecordplane.html little chip only had 16 bytes of ram, 256bytes of code, ran at 1MHz and done 2chn PCM decoding and 2chn PWM output. Parralax was using Microchip PICS for there chips and putting it in a fancy learning package. Looks like Microchip has the 80MHz procs. You can write programs in RISC or even in C. I wold imagine the port of PD would be a  nobrainer almost.  Atmel is another microcontroller that would possibly make a nice little PD stompbox brain. What  form of an embeddable host are you likely to find BSD in these days? I would look there first then shop around for something along those lines.