I know BMW Owners Club of America and the Harley owners clubs in the US have helped out people wanting to do what you are planning in the past.
Try making contact. Americans tend to be embarrassingly helpfull.
Or you could just pre buy a new bike from a dealer and work the problems through. Knew someone who did that a “few” years back (though via Canada where they are more user friendly to Brits.) and crossed the country on a 250 Honda XL without using Interstates.
Had a good time and ended up marrying a Canadian girl and just never came home.
Find a dealer via one of the US forums and arrange with them to buy a bike and then a pre-arranged buy-back with say 15,000 miles on it, no damage. You can get insurance using a PO Box address local to the dealer, and third-party insurance at least is VITAL, as is medical insurance for yourself. I’m hoping to be doing something similar in 2010, and it looks like all accommodation will be provided by US forum friends.