I once actually met a fellow hobbyist that was bragging about having adjusted a small plotting machine electronics into a bigger millingmachine by just adding some relais and ... whatever he was talking about.
sorry i'm no electrician so i didn't have a clue what the guy was talking about.
So it is possible to use the controllers and software used to drive the printer motors to drive some bigger motors.
Also the oprinters that i have raided until now only contained 2 steppers so i hope you aint using mach mill 3axes.(above mentioned plotter had3axes)
What i'm trying to say is that there have been lots of succesfull efforts to integrate printer electronics into a metalcutting machine but never any efforts to integrate metalcutting software into a printer.
My first guess is that mach wouldn't communicate with the printer drivers.
The difference in the above mentioned would be that when you integrate printer electronics in your machine you could just drive your machine with the printer software wich is offcourse beautifully adjusted an tuned to the used motors and drives.you could also use only the motors replace the drives and control them with mach3.
sorry i cant help but i hope these suggestions are correctly formulated and get you further down the line.