If I'd blown the led by doing what I did, lesson learned. It didn't blow because the battery was close enough to the nominal voltage of a 3w white LED. Let's also remember that a 3w LED can dissipate FAR more amperage than your standard super-bright, and this particular package has heat dissipation built into it (the aluminum core PCB it's mounted on).
So what did I learn from my test?
The LED will be painfully bright at it's nominal voltage and will definitely provide me the powerful momentary flash that I'm looking for, and I will probably want to current limit it a bit just for the sake of reducing it's brightness somewhat.
I will have to plan to make my multivibrator capable of handling the high current that the LED draws (3w/3.8v=~800mA), although I can probably get away with underrating the components slightly as it will only be used for short bursts.
I also learned that it's a lot harder to do a flash and fade than I'd thought.