Current law says the score at the time stands. Is that what you are advocating?
Otherwise, the tricky bit is deciding if the presence of the extra player materially facilitated a score. If it did, I think the score should in fairness be disallowed. That raises factual problems such as when did the player come on..
Awarding a penalty for the offence is no problem, though deciding where is non-trivial.
I think if the extra player has a material effect in a try being scored, then I don't have a problem with ruling out a score. I'll put a couple of scenarios to you...
A: a player scores a try, which you award, then the AR brings to your attention, a head high tackle by an attacking player. You consult the TMO and he agrees.
B: a player scores a try, which you award, then the AR brings to your attention, a an extra player on the field for the attacking team. You consult the TMO and he agrees.
Now for A, I think you would disallow the try and go back to the point where the tackle was made (the mark) deal with the offender as appropriate, and award a PK at the mark. If so, then why would you not do the same for B?
As for the mark, I think the mark could either be
a. a place that is representative of where the extra player came into the playing area which AIUI is officially the intersection of the half way line and the touchline on the technical area side of the field. So the mark could be 15m in from that touch on the half way line, or at the centre of the halfway way line
or
b. the place where the ball would next have come into play. With the disallowed try scenario, that would be the centre of the defending team's 22m. In a non-try scoring scenario, it could simply be wherever the play was when the extra player was brought to the referee's attention.