I have worked in television news for 20 years. We often have had need for hidden cameras or car mounted cameras.
I've mounted cameras in a lot of weird and wonderful places.
The easiest and smallest video cameras I've found is also one of the cheapest.
It comes in wired and a wireless versions.
I paid $250 for the wired camera and recorder/viewer. Samsung SC-105
The wireless version below is 2 or 3 times that cost. Samsung SC-210WL
Samsung Unveils Digital Camera with Helmet Attachment - SlashGear
The camera records onto an internal flash drive or an SD card.
The recorder/viewer also will work as a stand alone camcorder without the helmet camera attached.
I would recommend a RAM system for mounting to the riser just above the headlight. The farther up or out on the bars you get, the more vibration you'll get.
They will not have the image resolution necessary for good stills or crystal clear video. But they're cheap, small and fun as hell to use.