There's no problem at all putting a HID in a reflective housing that is designed for a HID. There are big problems with putting a HID in a reflective housing designed for a halogen. (While no one does this, it would be nearly as bad to put a halogen in a housing designed for a HID.) If you look at the LS headlight assemblies, you can see that the reflectors for the ones with HIDs are very different than the reflectors for the ones with halogens.
If you are going to retrofit aftermarket HIDs to a car with halogen headlights, the only decent way to do it is with projectors.