Why wouldn't the beagle-board, running real-time linux and EMC2 qualify ?

If you want to write your own real-time operating system, g-code interpreter, graphics drivers, PID-loops, etc. etc. etc. then please go ahead, but unless you are very talented and have lots of resources (time+money) it will be difficult.

To me it seems the real-time port for the beagle-board would be an excellent way to go.