Many US policies will cover it,
I've found this
not to be the case. I've ridden in the Irish Rally most years since 2000 and the problem of obtaining insurance has become increasingly difficult over the years, to the point where it is now
nearly impossible.
While my car insurance (State Farm), supplemented by insurance automatically supplied by my AmEx credit card, covers automobile rental, both explicitly exclude motorcycle rentals. I realize the question is about borrowing a motorcycle, not renting, but the same exclusion applies.
I own a motorcycle that lives in Ireland, titled and licensed to me. Since it is my own motorcycle I can get expensive "Green Card" insurance that covers whoever is named as a rider on the policy in any European country for a month (longer, if I pay more). The link in Bruce's post is to one company that offers that type of insurance, but note that it says it is "for your own motorcycle." The "Green Card" insurance I've used the past few years is issued by an English company so I have no idea what will happen after March 29 when the UK becomes non-European.
The insurance issue became a problem for me starting a few years ago because my younger daughter started coming to the rally with me so one of us has to be on a borrowed bike. Getting insurance for that has been an ever-changing, increasingly-difficult adventure. As it was explained to me, although there are various companies that issue insurance policies on old bikes (Carol Nash, Footman James, etc.) all these policies are underwritten by some massive conglomerate in Switzerland. It is they, not the individual companies, that determine what's allowed. Ten years ago my Irish friend just had to call his local agent to have me temporarily added to his policy for the bike I would borrow. That has ceased to be an option.
Thanks to the Irish Rally instituting a strict pre-1950 rule for bikes, and my Irish bike being 10 years newer than that, insurance has now reared its ugly head yet again. Both my daughter and I will need to be on borrowed bikes next year. So, I'm seeking a solution to this myself, but experience has taught me that it isn't going to be easy (if even possible).