I tested a few different etch-resists over the weekend with good luck, but the process itself still needs improvement before the boards will be usable.
The three resists I tried were Johnson "One-Step No Buff" floor wax, Krylon purple spray paint, and black Lacquer spray paint.
I made three test pieces of PCB material, one using each coating, then scratched each one manually with a nail, and etched. The wax was applied by pouring a thin stream over the top of the metal and propping it up at an angle on a paper towel to let the excess run off.
After etching, the floor wax was a clear failure, the coating was just too thin and failed over about half the surface. The purple Krylon seems like it worked well, as did the black Lacquer. Clear paint looks awesome but its hard to tell when you've sprayed enough on.
Also, one thicker coat seems to work better than two thin coats like you might normally use.
I still have a couple of problems to solve before the process will be more useful however: I need a wider scratch mark, and there's an annoying jitter in the tool during the first quarter or third inch of motion on the X-axis.
After making the test pieces, I scratched a full-size test piece using the matte black Lacquer.
Edit: Unfortunately, the etched scratches still do not completely separate the areas of copper, resulting in 100% shortage across the board surface.
Clearly I need to find a better resist and tool usage combination. The fine parallel lines still resolve clearly, but are also not fully clean. They are also not close enough to merge when etched. I think a softer resist might be desirable, so now I'm thinking .. what about melting a very thin layer of candle wax onto the surface with a hair dryer and then scratch wax but not metal with something pointy but not sharp, like a tooth pick?
Also, much hotter etchant may work better as well (this most recent board was done at room temperature and appears to be incompletely etched before the resist started to fade. Previously I used a double ziplock bag with a few TSP of etchant in a hot water bath with good results.