For PCBs I use a bit different of an approach. I have seen the pen version and the x-carriage mounted mill. But I don't think you can really get a good resist from a pen plotter approach (maybe, I haven't tried it but base that on many years experice etching boards with the toner transfer method) and your x/y resolution is limited by the nib diameter of the pin. For the mill I didn't want the mess or the hassle that comes from reconfiguring for this. So instead I made a spring loaded scribe that mounts with a single screw on my x-carriage. I coat the full blank board with a resist and use the scribe to scratch off the non-copper regions of the resist. Then etch as normal. This seems to work well and gives quite good results.