From Nick A., Progressive Internet Representative, in an email to a British traveller wanting insurance for a months ride across the USA on his own UK registered bike:
"You would be able to purchase a policy with Progressive with either an international or foreign country’s driver’s license as long as there was a permanent address within the U.S. where mail could be sent and the vehicle would be kept.
There would be a surcharge for having a non-U.S. license, but that would be included in the quote prior to your completing it.
Unfortunately, we do not sell short term policies. Motorcycle policies are currently sold for 12 month terms."
So sadly not much use for travellers passing through. Reports say they're good for US residents.