Couldn't the reprap be used to mask PCBs in its current form? I believe that plastics resist etchant solution just fine. After etching, the plastic could be removed from the boards with a cheap heat gun or maybe some solvent.
Here's what triggered my thought:
=========================
[
reprap.org]
Drawing / Plotting Head
This head is actually quite simple. It consists of a pen or marker as a drawing tip and a mount to hold it, It can be used to draw stuff in 2 dimensions. the third dimension becomes more of an "on the paper / off the paper" movement, and there is even no control board required for the head itself! One very useful aspect of this is that certain common art pens are etch resistant and this head can be used to make PCB masks directly on the copper board.
