I can't say for sure on light leakage, but JB Weld (not the rapid, or the putty, just the original) is liquid enough to soak into that crack a bit, is completely opaque after curing, and can even be scraped and sanded smooth after fully set (24 hours). I've got a couple old Bakelite cameras with JB Weld repairs that have stood up for more than fifteen years. In my work life, I've actually performed a repair on a sealing surface in a nail gun, where the repair had to stand up to 120 psi compressed air for hours at at time -- and no leaks, at least for the couple years before I left that job.
JB Weld is the stuff.