As the Seattle Sounders enter the quaterfinals after smoking Chivas USA, I find that five MLS clubs have lost during the single elimination tourney.
In fact in the round of 8 only
3 MLS clubs remain
The USL is guarunteed at least one team (Richmond USL-2 v Charleston) in the Semis and since Seattle and Charleston are hosting in the Quaters there is a decent chance one of them moves onward.
A second USL-2 (if MLS is = to MLB, USL2 = AA) squad is still alive and faces New England, but at Gillette stadium.