Yes, the Cygolite expilion/pace headlights seem light...and compact, fitting easily in the palm of a hand. As for use of these lights on bike helmets, I don't presently have a light mounted on the helmet, but I might consider one if there was something on the market, better suited for that purpose than there currently seems to be.

There may be something obviously wrong with it that hasn't occurred to my non-designer mind, but here's an idea: it just seems logical for a company using basically good designers, which Cygolite seems to, drawing up and engineering a double ended light...white headlight on front, red tail light on back. Adapting the same basic light housing components, reflectors, switches and LED's used for the headlights, I would think a nice looking light using a 18650 could be devised. Two headlight bodies (total length 8"), or...one full length one and half of another(total length 6") could make two halves joined together using a kind of lock such as that used by the expilion/pace headlight rear battery cover.

Using two headlight bodies together, logically would allow two 18650 batteries, one for headlight, one for tail light, allowing power enough to maintain current headlight run times; though maybe an 18650 for the tail light is more than necessary. Maybe the weight of two cygolite headlights mounted to a helmet is more in that location, than people would be comfortable riding with.