Just got a Metroflash Nocturna rear light for my helmet, but I'm having trouble making the velcro strap fit through the oddly-shaped vents. It does seem to clip quite handily onto the webbing at the base of the helmet strap assembly, though (between the two rear helmet reflectors). Is this an OK location? It's halfway up my head, so it should be pointed straight at onlookers. Or maybe i should be worried about an electrical short setting my hair on fire....

I'd be more worried about the light compromising the helmet. Without comment on the wisdom of wearing a helmet, it seems pretty simple that unless the light has been tested to not interfere with the helmet's performance it should not be put on the helmet.

That is exactly where I mount my 5-LED Yellow Blinkie; on the rear elastic strap between the two reflectors. Haven't had any problems, don't even know it's there. Once in a while I'll acidentally leave it on for a few hours and I have never noticed any heat. YMMV. Despite the 5 LED's it's an older not so bright flasher, so as not to blind drivers. Co-workers have told me it is very visible up high, which is why I did it. That's it in my Avatar Photo.

Lest there be any confusion, the helmet most likely hasn't been tested with a light, or anything for that matter, attached anywhere. It is only designed to protect within design standards. Attaching something like a light to the helmet is outside those standards and may result in more than a little discomfort in an impact.

If it is on the harness the same rule applies with the added hazard of that thing bearing the full impact and passing that on to your skull. Hmmmm, now that I think about it a permanently imbedded light in someone's skull might be a profitable side show at the fair.