Has anyone ever considered porting smoothieware to ChibiOS?
Pi’s are cheap and fast. (the zero is finally available and its only $5)
Chibi OS is a RTOS should be able to do Stepper signal timing perfectly.
@Josh_Rhodes on a PI I would go with LinuxCNC. There are realtime solutions for Linux, either patch the kernel or run two schedulers (one realtime and one not).
See:
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?RaspbianXenomaiBuild