I have an option to purchase a hobby multi-purpose device (lathe/mill/drill/grinder/cutter) which is manually controlled, but easily converted for driving by stepper motors (all 3 axis are controlled by turning knobs that can be replaced by gears, with convenient mount to couple each to a stepper motor). I have the right motors and can easily obtain drivers for them.
That is the way to overcome the worst problem of converting Prusa to CNC: the flimsy mechanics not able to withstand stress and vibrations of machining. Then I can connect the drivers to the 3 axis of Prusa's electronics, optionally connect some driver of the spindle to the extruder output (or just control it manually), and it seems the hardware side of the device is done.
The problem is the rest - adapting the software. RepRap family of 3D printers being open source means their software and hardware can be adapted. It's only a matter of how hard it is.
Does anyone have any experience in that direction? What would such conversion involve? Just recalibration to the new gear/leadscrew ratios, or something more involved, like editing the sources to get rid of all the temperature safeguards and the likes?
I haven't done this myself. But the temperature safeguards only apply to the 4th, the E axis for the filament. So configuring the right steps per mm would get you started.
The question is more what do you want to do with it and where do you get the G-Code to do that. You can not use a slicer to generate the G-Code for you.
But there is software for PCB Milling out there that should work with the configured Firmware.