Bontrager is offering a helmet model, the Circuit MIPS, that accepts magnetic mounts for GoPro & Bontrager fore & aft lights. Trek sez these mounts are tested to pop free ('breakaway') at a calculated force level.

I see some of our respondents above said since they've never crashed and hit the top of their helmet, they don't see the sense in a breakaway mount should they ride under a tree branch and snag their light/camera. We all get to make our individual decisions.