Here is a picture of one of six goes I had this weekend.
As you can see, when milling just to scratch the surface the resolution was perfect. When attempting to mill through the copper the bit would catch and dig right through. I put a piece of red tape where the bit went through to make it stand out.
I still think it may be possible to pull PCB milling off if you are willing to redesign the x-carriage, but I am against drifting this far away from the original Darwin design. For anyone wanting to go as far as redesigning the x-carriage, I envision mounting the milling head between the guide rails. In this way, if the guide rails twist, the bit should retract from the milling surface in both the -Y and +Y directions. A better long term solution would be to use a design closer to a McWire for milling / subtractive manufacturing and the Darwin for additive.
At this point I have gone ahead and ordered a stepper motor driver board from Zack's new shop. When I regain my motivation I will continue working on my extruder.