I have mounted the parallel port break out board inside the enclosure of my ScanTek 2000 (Denford Micromill). I routed the parallel port cable out to the previous DB25 (RS-232) outlet on the case and sealed up the back.
With the built in wire management trays, the job looks almost professional…except for the fact that I used a triangular piece of 1/4″ acrylic scrap I had for the mounting plate, so I had to orient the break out board at an angle. But since it’s inside the case, nobody can see it anyways. 
Outside the case, the only visible difference is that I took off the black “RS-232” sticker that used to live next to the DB-25 connector.
Here are some photos of the original RS232 cable:
Jay. Thanks for the detailed information on your project. It has inspired me to have a go at my Denford VMC 1300 PRO mill. This is a serious sized mill that uses the NextMove controller and a Baldor MicroFlex servo drive for the spindle controller (which is huge!). Helpfully, Denford post the schematics, so I should able to figure things out. The machine has spindle and feed overrides on the cabinet, for example. I think I’ll follow your lead with Mach3 initially before I try LinuxCNC
Good luck! I doubt you’ll be able to get the cabinet spindle/feed overrides working with a parallel port break out board without a lot of extra work, but everything else should be relatively simple assuming your NextMove ST controller is controlling the speed of the MicoFlex in the same way with a PWM output.