I'm not too sure if a heatpipe would work well at all. Two issues come to mind for me: whether or not the heatpipe's effectiveness depends on a particular orientation, and where exactly that heat is going. Bear in mind, though, that a CPU heatpipe is being used to transfer a large amount of heat from an exceptionally small surface area to a much-colder, often actively-cooled heatsink. Overall there's more demand than a flashlight has by far, with a larger temperature gradient.
Most of this is going to be written under the assumption that you want a pipe-style light, ie. Maglite, and not a light with more of a square-ish shape.
IMO you'd be better off just being more creative with the flashlight design. I'd imagine that plugging the light emitter into a shallow copper bulkhead that extends to near the head, where it mates with a series of aluminum fins, would be good for our purposes. Copper is well-known to conduct heat better than aluminum (it's just more expensive.)
Otherwise, or perhaps in addition to the above, you'd want a design that allows for a heatpipe or two to run along the battery tube. You'd either want a large cylindrical tube (Mag D) which would hold two series of batteries side-by-side and dual heatpipes in the dead space, or an oblong shape with whatever battery configuration is desired and heatpipes running however one would wish.
If orientation is not an issue (I'm fuzzy on the exact physics involved here) then this might be a decent solution, but you'd want the body to have deep knurling, ribbing, flutes, etc. to increase surface area and help with surface-to-air heat transfer. This is important because heatpipes are a passive solution (as opposed to Peltiers, which might be an interesting (if grossly energy-inefficient) solution for hotrod lights) and their effectiveness relies on the "to" end being cooler than the "from" end.
On the subject of orientation, you might need to at least ensure that the cold end of the heatpipe is not positioned lower than the hot end at any point, or a certain minimum angle might be necessary. A pipe-shaped light might find a heatpipe useless if stood on end, lamp facing up, as a result of this.
If you go with a square-ish light, you can just use a series of heatpipes in a star formation of some sort and be guaranteed that at any point, at least one will be pointing up.
I hope this all made sense. :shrug:
