The software supplier's 'requirements' indicate that the HP printers need the bar-code DIMM, yet the OKI printers appear to be able to use a downloaded soft font (albeit downloaded to a permanent 'disk' resource).
I see no reason why the HP printers could not use the same downloaded soft font, stored in RAM, the only caveat (as mentioned before) being that the font download has to be repeated every time that the printer is powered-on; or (depending on how much you can tailor the 'proprietary system'), the font download could perhaps be 'prefixed' at the front of each job generated by that system.