Yeah, that seems to be the sad state of the situation. So since it is hard to get hold of firmware, chances increase that it is old. Did i understand you correctly that you had tried with Postscript some other way, and it didn’t work? Maybe contacting support could be a good idea.
If you have it working with CUPS on a Linux machine already, you could share the printer from it, and use that. But that’s of course no fun with a nice printer like that.
If you have the SDK set up and feel comfortable changing a little code, you could change ippprinter.cpp at roughly line 134, so that it does the following unconditionally:
_additionalDocumentFormats.append("application/pdf");
_additionalDocumentFormats.append("application/postscript");
// and maybe even add this for good measure
_additionalDocumentFormats.append("image/urf");
If that works, your printer is the first confirmed case of something that supports reasonable formats but with no way of detecting it.
We all keep learning. Looking more closely at that printer, there are some hints about it supporting Postscript… but nothing super-conclusive. It seems that they are hiding it behind something they call BR-Script. Seems i have some more reading to do.