I agree that most other printers (except for a few high-end models) would not include any form of permanent storage by default, so it would seem rather pointless to just buy another printer, especially as different devices may not support the same types of storage.
I'm also not sure how device numbers are assigned - probably automatically done, and different for each model, although I'd think that 'disk 0' would probably always be a built-in hard disk; not all devices accept other forms (e.g. DIMM) of permanent storage.
As Jim suggests, the contents of the existing PJL file could almost certainly be (relatively easily) modified to cause the font to be stored as a 'permanent' downloaded soft font in RAM, but (as mentioned before) this would still mean that the download would have to be done every time that the printer was powered-on , and (in certain circumstances) perhaps more frequently.
We could help with this, but we'd need a copy of the file, and perhaps also details of how the 'proprietary system' selects the bar-code font (and then reverts to the text font(s)).
I don't use WebJetAdmin, so I'm not familiar with the printer resource management features that it may (or may not) offer.
