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.
breakaway sounds really smart. last summer my helmet camera hit a tree branch while I was bushwhacking a remote desolate trail. the thing broke right off. easy fix & a minor inconvenience. if I was knocked out or disabled I doubt my body would ever have been found. that would have been a long agonizing death
fast forward to the :40 sec mark ;-)