Hi Sean, I did just this with a HP7475A a couple of years ago. It works
tolerably well, the lumocolor 313 ink (I used black) will etch through if
the etch goes on too long though. Use a strong solution, keep it hot and
remove the board the instant it is finished. I found I could reliably do 30
thou tracks with it, not exactly state of the art, but quite useable.
The 7475A moves the pen on one axis and the paper on the other- I first
plotted the layout on paper, then (without moving the paper) used a
gluestick to stick the blank pc board to the paper over the top of the plot,
then plotted it again with the lumocolor pen. I used the slowest pen speed
and accelleration as the pen had a tendency to tilt over at the default
accelleration.
I had to cut a piece out of the cover on the plotter to allow the pen to
move out of the carousel as the '313 is a lot taller then the standard HP
pens. The plotter software should allow you to specify which pen(s) to use,
just tell it to use pen 1. I used protel easytrax, it's free and works very
well.
You used to be able to buy refillable pens and etch-resist ink for the HP
plotters, but my local suppliers claim they can't get it any more. I did try
an ink-pen but the weight of the pen was too much for the pen-holding
"claw"on the plotter Pity, it makes an excellent etch-resist it you don't
mind 50 thou tracks.

Cheers, Ken Johnson
vk7krj@southcom.com.au