The WODs vary so much, I wouldn't think you could track it as an activity since the amount you burn will vary depending on whether it's an olympic lifting or a chipper or whatever day.
I wear a Polar HRM to class. If I was tracking right now, I'd use that to input into the calorie burn MFP. It really varies widely depending on the WOD and how intense the warm-up/gassers are.
I usually count it between 300 and 400 calories depending on the intensity and length of the WOD. I don't really trust MFP for calorie estimates on most things, I almost always adjust whatever it calculates . For instance, in my experience it gives too many calories burned for cycling, so I use the metric of about 35 cal/mile (for flat rides).