A few months ago, we flew Delta from HND to SEA. Since we used miles for Delta, we had to purchase tickets separately from ITM to HND. Did that with JAL a couple of months before departure and paid half the price of a shinkansen ticket. JAL and Delta have no shared agreement. Regardless, JAL sent us to the international transfer check-in desk at ITM and checked the bags all the way through. In addition, they allowed for the international luggage standard rather than domestic, so we didn't have to pay any extra fee for luggage. Sure enough, next time we saw the bags was in Seattle. Very pleasant.

I've since heard from others that they had the same experience with ANA or JAL.