There are OEM machines that have MPG wheels mounted in traditional locations, and do allow you to drive an axis both for set up and also mid job, all while the control automatically recalculates a running process (the brand I have witnessed doing this escapes me at the moment).
On the DIY level, this individuals experience should be the center of great discussion in my opinion: http://www.cnczone.com/forums/bencht...rol-g0752.html
I hope to utilize his methods on a lathe of my own, but I will have to learn a lot more about Arduino programming to use it as his code was dialed in around some rather specific brand unitized stepper driver/motors.