I seem to remember that a non-PostScript model of HP printer will work as a PostScript printer - as long as you fit the correct PostScript SIMM in the printer.

I remember a few years ago owning a HP 5 Color LaserJet printer (the non PS model - identified by having the "M" in the title - i.e. HP5M) and when I fitted the PostScript SIMM in the memory slot it then worked as the HP5M PS Colour Laser Jet printer and printed documents out in glorious PostScript quality - and on that basis the HP6 may be possible to "enhance" it with PS capabilitiy if you can acquire a PS SIMM for the memory slot of that model.

Bear in mind that this is NOT a guarantee that this will work, but for the cost of acquiring a PS SIMM for this printer it may wel be worth the outlay to try it out, and if it does work, report back here for others to be aware of.

[EDIT] This seems to be what to look for - if you want to try this option

HP-LaserJet-6P-Printer-Postscript-Level-2-Module-1MB-Memory-C3987-60001