So, I just answered this question in the aspiring JETs forum, but in case you never check back there:
I have been a part of a yoga class here since roughly September, it's just one town over run in the town's community center by a resident obachan. Although I enjoy going to the class and getting to socialize a bit, it is quite different from yoga at home and I feel like it mostly focuses on stretching with only a few true "yoga moves" worked in. The general format and structure of the class is pretty different, too, and I get kind of irritated sometimes when I do a certain move with a little bit of a variation, especially when I know what I'm doing, and I immediately get corrected to do it the way everyone else is.
So what I'm trying to say is although this class isn't exactly what I'm looking for out of a yoga class, and I do prefer yoga classes back in the States, to me it's still worth going to. Like I said, it's a good way to socialize with other Japanese people that live here/other ALTs, I still feel good after each class, and it's teaching me a lot of patience (and I still do yoga the way I want to at home). And as one added bonus, at the end of each class we give each other little massages, which is very nice! We also sometimes have small parties, so it really makes me a feel like a part of the community. So even if you can't find a yoga class that's exactly what you are looking for, I highly recommend still going to one.
Also, yeah the class is run totally in Japanese, but I can usually follow along by watching what the sensei is doing (and honestly she doesn't give too much instruction anyway, at least compared to back home). It also only costs me 3,000yen per six months to be a part of this class, so it's super cheap.
OOOH, related to this, I actually do go to a Zazen meditation class once a month. It's in a temple run by a Buddhist priest, and the class consists of sitting in a tatami room in a seated position on a little cushion for 45 minutes. He gives almost ZERO guidance, except the way in which to sit down on the cushion, and afterwords we always have a potluck, drink, and go out for more drinking/karaoke. So there you go.
...And yes, I live in the inaka.