Not a bad idea considering United draws almost as many people to its games as the MLS Cup. You'll probably get the 20k United fans plus whatever. The MLS Cup only drew 22k this year
...but the 22k was in fact a sellout crowd (it's a single-use soccer stadium with only ~22,000 seats). Last time DC United appeared in an MLS Cup final at RFK, it also sold out, and last time the MLS Cup was at RFK (but without DC United), it still drew 40,000. I'll be going to the MLS Cup final regardless of who is playing DCU 
I saw a re-run of the first MLS final at RFK and it blew my mind to see it packed for a soccer game. Its pretty cool. I'd like for soccer to catch on big time here.
I dont know if you saw it, but there is a documentary called "Once in a Lifetime: The Story of the NY Cosmos". For 4 years they were the biggest draw in NY. Bigger than the Yankees. One guy in it said they were as big as Man U and Real Madrid. It blew my mind (I was not born yet). They were regularly selling out Giants stadium for regular season games. You shoudl check it out
