To get around that, market it as an International brand of football, not as rugby. Stress the parallels to American football in every way possible: make sure every game is played on a uniformly marked field with solid, numbered lines like Americans are used to, replace rugby terminology with American terms like touchdown, report game stats in American football style, etc. The key to breaking through in the US is getting American football fans interested in the game.
We don't need to market it football because Americans, regardless of what the rest of the world thinks, are not that stupid and understand what football is. They will either think that a) "foreigners don't play good football so I'm not going to watch it" or b) "this isn't football, why'd I waste my money."
But if we held the cup and marketed it as the best rugby players in the world coming to the states, and showed games on television in the states in the runup to the cup, we could get a great turnout. It isn't going to rival any homegrown sports, but I do believe that there would be 20k+ at every single game. Britain and Australia simply lack the population to support numbers like that--even though it would be a minute percentage of the American population, even miniscule support would blow what Oz and the UK get out of the water.