Bo2 would be perfect for Intermediate here, didn't we agree on that a while back? It's even more fair than Bo3 as long as you play both rounds on islands or both in caves, and the only thing that would have to be done is to allow reporting 1-1 draws when reporting Inter here on TUS. Simply have that single report affect both players' ratings the same way as if they each won one full game, so the higher ranked one loses a bit of points and the lower ranked one wins a few.
Isn't it essentially equivalent to reporting both rounds as separate games?
Not really, no. If Bo2 was the default and players could agree to play out the full Bo3, winning 2:0 would still only count as one game of Intermediate won, same as winning 2:1 would. If every round was reported separately, on the other hand, a 2:0 win in rounds would count more than a 2:1 one, and that kind of goes against the way the scheme works, seeing as equally strong players are expected to win the round in which they get first turn. Not to mention being able to report 1:1 draws would take away the hassle of having to report the games in the correct order, as well as keep the replays of all rounds played on the same game # page, clearly indicating that they were rounds of the same match, played under equal conditions (as far as starting first and map choices go).