I don't wear a HRM to classes. But I have a Polar that I wear when I'm working on my own. I have a pretty good idea of how much I burn at what level of exertion, so I either estimate my classes or just log them as "1 calorie."

I've read posts on another board where people have opted to wear their trackers folded inside their belt on other unusual places to continue counting their steps during training. I have a Fitbit Flex and while I did wear it to class a couple of times, I'm not a big fan of doing so. First, especially in arts that involve throwing, falling, etc. your step count is almost guaranteed to be off and likely your calorie burned cout. You also run a fairly high risk of landing on it and either potentially breaking it or hurting yourself or someone else.

I do agree that the default calorie loss listed in MFP most likely isn't very accurate, but personally I'd rather take that and use it as an estimate instead of risking injuring myself or someone else. If you do want a set number, maybe hide your tracker somewhere (in your belt?) for a class or two and try to get an approximate calorie count and then just key it in manually from that point forward. I would just be extremely careful since especially doing judo you'll have a higher chance of breaking it or hurting yourself.

I use a scoche Rhythm+ which wraps around the forearm - it's pretty low profile, sweat resistant, shock resistant.. I seldom use it during partner work, but when I do, I turn it inward to keep it from getting directly hit during sparring (Taekwondo)... I find it pretty useful for tracking calories (albeit with a grain of salt for accuracy of burn rate), and for working on endurance training.