I may stand to be corrected here, but my understanding is that the BMI limit is imposed by the AIRLINE, and not JCAB. At least I have on very good authority (in writing from our Japanese airline) that in fact our limit is a BMI of 30-thirty. This is further supported by the fact that I am a fat bum myself at BMI 28 - and I just passed the JCAB medical a couple of months ago.
Cheers all - Haywood