My suggestion would be to get a DSP from Microchip or TI or use some ARM core (all have good C compilers and can do the heavy lifting needed, most are <15USD) and work out the math to convert XYZ coordinates to UVW axis locations using vector math. Run this on the DSP. Anyone with a good grasp of geometry and a bit of motivation should be able to come up with the equations for this. Sorry that it's not much direct help, but using a powerful core would make it possible to be compatible with the RepRap host side software.