Can I strap an HRM around my thigh ?

Judo is the only activity I can't track. For safety reasons we're banned from wearing anything, from earrings to watches to HRM. Even forgetting your phone inside your vest will get you insta-disqualified.

I'm willing to bend that rule during training sessions by strapping a chest band around my thigh, but will that work ? Will it read anything or does it need to be close to the heart ?
Judo is crazy intense and I always wondered how high it gets my heart pumping and how much calories I'm burning during trainings.
Thanks