Alternatively, should I consider converting the image data to some standard, which I could dump directly to the printer? What would that standard be? Can you open a ethernet printer port using open?

I have had good luck taking .ps files off my Unix systems and running them through a batch file which just copies them to the device:

@echo off
rem This batch file is used to send pre-formatted PostScript documents
rem to the printer \\SERVER\PRINTER. It is intended to be used as the
rem "print" option for .ps files.
copy %1 \\SERVER\PRINTER
[download]

It occurs to me that if some module exists to create PostScript output (or whatever language your printer understands) from the images you are rendering, that you could just open the UNC name of the printer as a file and write directly to it.