Done. The card just arrived today!
Working very nicely so far - can download plots from spectrum analyser and dynamic signal analyser. I am using a demo version of PrintCapture on Windows to get everything working. Next step is to do the same on Linux. I need to figure out how to put the interface into "listen" mode and capture the HPGL data into a file. Something for the weekend!
Agreed, I was not really thinking of making one myself. I thought someone else might have something on sale that was compatible and cheap...