For information, on the French cyberstratege site, there's such a thing:
http://www.cyberstratege.com/forum/v...php?f=7&t=2134 for those who read French and wouldn't know that site.
They limited themselves to 4 player games on a single map, with random nations and random era. I didn't check the scoring system because I can't follow with the rhythm of the games (daily) during week-ends. Anyway the scores are here:
http://www.cyberstratege.com/forum/v...php?f=7&t=2211 . It's interesting to notice that after 5 months, 7 games were completed, and all players completed at most 2 matches.
Therefore, I think a system where your ranking depends on your opponents would never work (unless everyone plays for a century or more, or the ladder games are all blitz games).