Yeah - there's that - I didn't think about that. It so happens I have a carbon bar (Easton), which goes into a carbon stem - which connects to a carbon fork, which is attached to a carbon frame (Trek 5200 road), which rides on (Head) carbon wheels. Not much of a heat sink, doesn't even get warm on a summer day in the sun.
The other thing to think about is that off-road riders usually want their light helmet mounted so it points where they're looking since the direction the handlebars are pointed and the direction the bike is actually going often bear little relation to one another. Besides, heads don't make good very good heat sinks either - not intentionally anyway.
By the way, nearly any bike headlight or commercial fixture that takes an MR11 bulb can take a Luxeon with 30mm optics with some fiddling to mount it. I made a helmet mounted light from an older Vistalight aluminum head I begged from a friend this weekend. It was designed to use an MR11 10w halogen bulb and the Luxeon 30mm optic fit perfectly. I used a 2000ma 7.2v NMIH race car battery I had. It works OK, though not as good as the 12v one I made because I don't have the current/voltage right so it's either too bright (too hot) or too dim (anybody know what I SHOULD use to get the best output?).
I really wish I were better at the electronics end of this. Maybe I should ask my cat, he probably understands it better than I do. I only have a couple of Luxeons and at the rate I'm going I'll probably toast at least one of them soon with my fiddling about.
Another thing; You might want to check out this link - they have all sorts of lenses - colored and clear for different needs, all are designed for MR11 bulbs so they should fit the30mm optic as well. Some of them might be useful.
http://eclecticlighting.homestead.com/optionallenses.html