but these are difficult to coordinate without both having a Japanese cell phone

Yes... why oh why does Japan have to be so difficult? It has only been recently that I could even get my US cash cards to work there.

One (somewhat risky) tactic would be to bring a pair of US walkie talkies along. The unlicensed band in Japan may differ from the US so you will want to keep the transmissions short. I can research this if you want. Or, maybe we could even send you a pair from Japan so you don't have any risk of interfering with anyone.