I think it would be sensible to add a case for it in tos.c. I feel like it would be only a few lines of code for a programmer, but a very complicated instruction for an end-user to learn how to modify their TOS. Is there a checksum or something in the header of this known ROM that would make detection easy?
PS: There is a dutch TOS 1.04 available in the internet which seems to use
the wrong language code in the ROM header ... so that does not work with
the symbolic mapping in Hatari yet. No clue whether we should add a hack
for that in Hatari, though, or whether we should recommend the users to fix
up the bad value in the TOS header instead...