@gnomadix,
Glad to hear we got your gears turning! I haven’t built a RepRap myself, but I’m guessing that the basic idea is identical: three motors for three axes of motion, plus maybe a fourth motor to run deposition. If you could add a deposition head to the CNC, potentially you’d be able to run both additive and subtractive jobs.
The limitation here is likely current. We use pretty big motors – see the HobbyCNC documentation for more info (you can get away with the smallest ones on that site). I’ll be that the RepRap driver can’t handle that much power.
If you’re feeling adventurous, it would be possible to re-engineer the lil’ CNC to work with smaller motors. This would involve adding gearing at each axis to multiply the torque of the motors. Z is already has a decent ratio due to its ACME screw, so all we’d need to mod is X and Y. Check out the downloads page for CAD files and more info.