It looks like the MPG101 uses the parallel port, which is deactivated if you are using an external motion plugin. So I don't see how it would work with that limitation. It has a plugin with it as well, from what I read. But I have not seen on or tried to get one working, so I can't say for sure.
The MPG4 is wired the digital inputs and a spare (or secondary) encoder on the Galil ICM. So the MPG is actually controlling the Galil instead of Mach. The MPG configuration is done in the Galil Plugin configuration dialog. This makes for super smooth MPG operation! You will also have to write a small brain that takes the digital inputs and makes them do the right thing in Mach (axis selection, step size, etc...)
Steve