About the closest you could get would be the US ratings of the World Cup Final vs the championship games in other sports. And even then, you're talking about one game that is not going to have Americans in it and is rarely going to be in prime-time compared to for the most part, games in a series. How do you compare? Maybe if you have a game 7 to look at. I don't know, but comparing MLS to any other American sports league is just flawed.
Edit: now that I think about it, you could probably draw some reasonable comparisons between US WC Qualifiers and regular season games from other sports. And US World Cup matches to playoff games. Maybe...