The current Bell/Giro helmet has a mounting strap for a rear light (a few years ago this model did not have the strap). Do note that unless you mount a really broad-beamed, bright light or a self-leveling one, a helmet mounted rear light may not always direct the beam level and backward where you want it to go.

Recent review of Citi helmet.



One self-leveling light is the Planet Bike Blinky 3-H which supplies its own helmet mounting including strap:

However, it is not as bright (and may not have as good a flash pattern) as their Superflash model.

Undoubtedly, there are other helmet models that come with straps and you may be able to make your own mount.