I read your post... do what you do now with your portsmouth correction. I can't see any changes.

you look up their base boat and start subtracting for corrections. oversized main = - 2%... spinaker = -4%. You are not trying to ACCURATELY rate them... you are rating them so they can race fairly in that regatta based on the information they provide. (If they expect you to walk the beach and spot their modifications as they push off the beach... well that is a different problem all together)

When the sailor cares enough to pay attention... he can go get his stuff measured... till then the subtractions are fair
If he ever bothers to get a measurement certificate ... he might do better on the race course.

He knows he has a bigger sail and should be faster... It becomes his problem when he wants it to be. It's never yours (you just subtract for each modification he reports) or the fleets problem (the subtractions are worst case hits) If he reports no modifications... the usual protest will solve the problem.

Some events will be more rigorous and they would require a measurement certificate for one off or modified boats. Currently the alter qualifier does not allow these boats to race.

I can't see your problem it's no difference in procedure