Hey, I've been tinkering with the same idea. Not for SAR, just the rotating shift, industrial work site my wife and I have to deal with. Some discussion of this topic in this
thread.
Our work hardhats are run-of-the-mill Bullard's. They perch above your noggin, hanging off the inner suspension straps. This is typical of all or most industrial hardhats. The "perching" method gives airspace between the helmet and your skull, which is good from a protection standpoint. But it also means that any weight attached to the hardhat shell which is not balanced causes the hardhat to list.
If your SAR helmet is more like a Petzl rock climber's shell, or a bicycle helmet, you'll be less affected by imbalance issues.
Does your hardhat have the slots above the left and right ears? Petzl (EDIT: Pelican, not Petzl, sorry) makes a couple of clamps that plug into the standard hardhat slots. I think they're kind of dopey-looking, but they might work for you. Their clamps aren't generic; some are distinctly oval-shaped to accomodate their flashlights although I imagine similar lights like some of the Streamlights might fit.
As described in the above thread, I disassembled a clip-on faceshield and kept the little plastic pivot points that clip into our hardhats. Using a little strip of stainless sheet metal I created a bracket for a small AA flashlight, but the imbalance created by this small flashlight and bracket was intolerable. Looks like I'll have to make more brackets, and either load up both sides of the hardhat or none at all.
Bullard sells goggle clips for their hardhats. The goggle clips are nothing more than a bit of hard plastic shaped into a clip with some strong adhesive on the back. You stick two or three of these onto your hardhat, and the clips should hold onto just about any headlight that uses a headband. Seems to me the Bullard clips (or something similar) might be the simplest solution to your situation.
