That's a really cool idea. Only problem is that I would change the victory formula a bit. I think the only important factor is Provinces. The others shouldn't really affect your ranking. If someone gets lucky and gets really really nice sites in his first few provinces and hordes a lot of gems, does that mean he deserves to be ranked higher than someone who went out and conquered a lot of land but didn't find any decent sites? Someone shouldn't be forced into the magic searching game if they have a strategy to conquer the map without using heavy magic. Factoring gems/gold will weigh the luck of your starting provinces too heavily. People would lose just because they were surrounded by low income/low gem provinces.
I would just set up each ladder for each map, with a random nation opponent but at the constant Impossible setting. Actually, something like Aran with one player against 4 or 5 Impossible AIs would be a better test, unless you find a good map that is made for 2 players. If it's just 1 player vs. 1 AI on a big map then they may never meet and it's like you're just ranking a player's ability to conquer neutral provinces.
Well, I suppose you can just set the rules and people will play by them. If you make a huge map with 1 AI then people will do well with nations that do well against neutrals out of the box. If you weigh magic sites heavily in the victory equation then people will pick nations that search magic sites easily, and so on. People will just game the system no matter what you set it as