"the assembler must have a table somewhere according to which it decides to accept or reject an address"

Certainly looks that way! :-(

"Maybe it's in the form of a file you can edit?"

If not, then this assembler definitely needs to be thrown into the bin pronton!
Otherwise you will be continually coming up against the same problem with all the "new" SFRs on all the "modern" variants.
Rather than keep bodging it, better to bite the bullet now and move to an assembler that does support the very wide range of variants available today and, therefore, tomorrow as well!