One question: was it dark enough that you really had to use your light? If so, how did you hold it? . . . if there's a "best" way to illuminate a wound that you're working on? . . . Or do you just hand hold the light to survey things first, then put it down on the ground to provide just enough light to go to work?
There was some ambient light, so it wasn't pitch dark. I just hand held for my initial assessment and set it on the ground to do interventions. There is an upconverted MiniMag in the kit, but the Fenix is my EDC, so I never touched it. I actually pulled the Fenix out at first to see into my kit. There are usually enough bystanders willing to pitch in and hold a flashlight for you. In this case, I set them to cutting the guy's clothes off.
I have some headlamps, and have used them in the past. They'd be great in a FAK. My kit was assembled/evolved from whatever I happened to have on hand at the time, with an eye to keeping the size reasonable, hence no BVM.
Ideally, you want intense spot and flood light to evaluate and treat injures. Ever notice the huge overhead and stand lights in every ER? More is better. I think your idea of a headlamp is probably the best compromise for a FAK, with maybe a low power cheapie for checking pupillary response.