I don't know offhand, but if you can find out the town or towns they'll be visiting, we could drill down in the "go hunting" pages to see what release zones are there. [Someone who reads Japanese might have a better shot at figuring that out, though!]
The LibraryThing site has a "little libraries" search option that sometimes turns up LFLs and/or OBCZs; again, it helps to know the name of a town, and the listings may not be up to date, but it's worth a shot.
I don't know if Japan has its own version of book-swap shelves anywhere; if so, that'd be another option.
And, my favorite choice when visiting far from home: look for scenic parks, statues, public artworks, etc., with handy benches or ledges nearby, and leave books there. Great photo opportunities, and it often seems that books found in places where people don't *expect* books have a slightly better chance of getting journaled.
Thanks, he's left for his trip and I'm not sure which areas he'll be visiting. I did 'go hunting' before he left. Sent him off with a book and I'm sure he'll find the right spot to release it. I think he was hoping it would be a good way to start a conversation with fellow travelers or locals along the way.
Since our travels are limited I always try to send some of our books with customers and friends who are traveling and willing to release them along the way. It's always fun for me to see where they land.