Just a quick update - mainly photos of progress.

I made a proper frame to keep the axes perpendicular, and control the height of the laser. I'm currently using an IR laser I bought, with adjustable focus. The laser driver circuit is controlled by a relay module (I was in a hurry), and the rest of the system is still the same - two easydrivers and a bare-bones arduino with GBRL. The test pattern below is the first usable output!

It took some trial and error to get the traces durable - the first few tries brushed off too easily. The trick was getting a thin regular coating of toner (very runny slurry of toner in alcohol - it should dry in a few minutes) and making sure the laser was focused properly.

Here's the board being prepared:

and the sink after a few spills and some silly mistakes:

NB: There are some health concerns about the toner powder. Please clean it before it dries and try not to breathe it in. It is just plastic, but because it's very fine particles it can be problematic.

Another shot of the XY part of the system:

The pic of the test pattern is taken using a DVD drive lens on my phone. The silver thing is a USB plug, and the boxes are (left to right) 5mm by 5mm, 5x0.3, 5x0.5, 1x1, 0.5x0.2 and 0.5x0.5. I filled the two largest using a permanent marker, and went a bit over the lines. Also, there are little 'serifs' due to lag with the laser switching system. I can't actually etch the test pattern until I get my hands on some vinegar, but I washed and rubbed the board quite vigorously and the traces held up, so I know from previous experience that there shouldn't be any trouble etching it.

Next steps: Tidy up the drive electronics into a box (with a fan), etch an actual board, make a decent video and figure out how to get a consistent coating (maybe a spin coater).