I'm thinking Aisthesis has the right approach below Louie. He uses your same reasoning but considers all equally likly 1326 possible starting hands. Here is a link to Hand Rankings accourding to Heads Up matchup.
http://www.gocee.com/poker/he_ev_pe.html
Interesting that J5s is on the bubble heads up.
I guess I agree that in a 3 player game the Bubble would be the hand ranked in the top 70% like you say. I'm not suggesting you think this but for others reading here, that's not the same as the hand that wins 70% of the time against a random hand when taken to showdown.
Thanks,
PairTheBoard