I figured the info about CCs not being widely used in Japan was probably erroneous. I asked the small ryokan where I'm staying whether they accept them, and they do (even though their website is still stuck in the early 1990s, apparently their payment handling has caught up to the 2010s). In a country as obsessed with technology as Japan - at least, that's the stereotype! - it's hard to imagine that credit card use is rare.

Just to be sure, we'll withdraw enough yen at the airport to cover taxi fare, and if the taxi takes CCs, we'll just keep the cash around for incidental expenses.