I thought about that, although I'd prefer to stick with the optic since I normally carry a separate light for flood.
The head is specifically designed to hold the 30mm optic, lip and all. The bottom of the lip presses against a thick rubber o-ring that sits in a groove that's recessed in the head. The top of the optic presses against the lens which presses against the bezel. Water cannot get in because it's sealed out by this optic to head o-ring. It's similar (but not identical) to the way the regular head maintains waterproofness, where water could conceivably get in through the bezel, but go no further.
To use a reflector, you might be able to get 2 thin o-rings, and do it m*g style by putting one between the lens and bezel, and another between the bezel and head. However, I don't think that would work all that well. I would not recommend that someone purchase one for modding to a reflector, until they can first get an exploded diagram and see what they may be getting into.