Perhaps new upgrade boards could be manufactured.... I have the schematics for the MacGusto II board in my 512k (but we would have to ask ddenowh (one of the original creators of the board, who sent me the parts for mine) if we made a carbon copy of it) However as that requires substantial modification of the 512k's logic board most purists would disagree with it
But I would think it would be possible to make a MacSnap style solderless upgrade (assuming the macsnap scsi cards don't require any soldering? I know their ram cards don't) providing the NCR5380 scsi chips can still be got hold of