In the end, after much swearing and throwing of large objects at these ruddy printers, we built a virtual XP box to share them and used a sketchy driver (old original HP inf, new HP dlls that don't crash). We turned off bidirectional support on the ports and shared them - no problem!
Conveniently forgetting that XP doesn't allow more than 10 concurrent connections, of course.
Built a new Server 2003 box, no updates with anything to do with printing, add printers, sketchy driver on, turn off bidirectional support, share them... done! I haven't heard of any further problems.
Oh except that the mirrors on all the 2600s are covered in ruddy magenta toner so the colours are ridiculously faint.
The driver I used is available if you want it, but you know you shouldn't take sweets from strangers, especially on the internet.