I've never used a LuxV in anything - do I NEED to isolate the slug from the cathode? I know the datasheet says that I do but I've heard of people running the Lux3 without proper isolation when only running a single LED. I don't see how the LuxV dies (dice?) cannot be isolated from the slug as there are 4 of them in series-parallel anyway.
If I don't need to keep the slug isolated from the body of the flashlight then designing a solid copper heat sink will be trivial. At 350ma I'll only be running the Lux at around 2W, so it should manage fairly long runtimes if it is held constantly. I might switch to a simple resistor in the tail or try and find a low side regulator that I can fit in the tail to keep the solid mass as large as possible in the head.