Hi Richard,
in the current vewrsion of GRASS a Deskjet Driver HP550C is
integrated. Look at
src421/src.extra/devices/
src421/src.extra/paint/ (I can't remember which one)
You find the source package at
http://www.laum.uni-hannover.de/iln/grass/grass42/
and soon on Baylor's site.
Many greetings from
Markus