Helmets with dedicated mounting system for lights, cameras etc.? I have came to the conclusion that headlights work much better for me for cycling than handlebar-mounted lights. This is because:

A headlamp illuminates where one is looking, not where the bike is turning towards.
The angle under which the light falls on the trail is much improved. When handlebar-mounted light illuminates the trail, the holes and irregularities look bottomless, making it difficult to select a safe line.

Suppose I am considering buying a full-face helmet. Are there models, that are explicitly designed to have a light attached to them. I imagine some kind of rails on the side or in front or on the top.
Bonus question 1: recommended way to mount a conventional headlamp (one which people put on their heads) to a conventional helmet (skateboard-type or full-face)?
Bonus question 2: any lights, that are supposed to be mounted on the helmet, but their power supply to reside in the backpack?
The only helmets with built-in light mounts I know of are by Bell, which can mount the Blackburn Flea. I don't know if any of them are full-face. The Flea is pretty useless for offroad riding, it's too dim.

Lights usually mount up with velcro straps through the vents. When I was racing my MTB after dark Ay-Ups were common - small, extremely bright, and a remote battery pack that can be put somewhere comfortable. Not cheap though. Most lights with separate battery packs should be able to mount to a helmet. Check out Bikeradar or something for options.