> Poseidon recognises it straight away.

Poseidon just displays what the USB device claims as its name. That's far from recognition.

> there are no specific drivers for it as far as I can see - nor even any
> vaguely close drivers. Are there any options for this printer at all?

According to HP, this printer speaks PCL3GUI (and PML), which is not compatible with "normal" PCL. I guess you can't use this printer with MorphOS.