I don't think there's a solution for that within Mp3tag.
It would require being able to access the tag content of the previous file in the selection.
Something that would conceivably work might be if Florian adds functionality like $previous(TAG) and $next(TAG) to refer to the TAG value of the previous selected song or the next selected song, but you'd have to create a feature request for that and it might be very complex to implement so there's no saying if he'd actually do it.