What's the likelihood that it's a hex dump one of these?

* https://github.com/virtualxt/virtualxt/tree/develop/bios

Because if you look back far enough you'll find that this is based upon something called fake86 written by a Mike Chambers in 2010, of which there are many half-written derivatives including this:

* https://github.com/lgblgblgb/fake86/tree/master

obviously replicated here:

* https://github.com/xrip/pico-286/blob/0d2c76ba51572addaeed8a...

and which loads a pcxtbios.bin file in several flavours, including but not limited to:

* https://github.com/lgblgblgb/fake86/tree/master/bin/data