FWIW, I took a bit of time and played with my Trustfire as test and the velcro wraps that I use to put it on my helmet.

I can put it under the NDS chain stay, but the beam is partially blocked (not much though) by the QR skewer and it's going to get awfully wet in that location. Tried lashing it to the side of the top tube, but the tube slopes and all of my control cables are routed along the top of the tube- they rub on the strap.

I think I've found something that will work- I can lash the light to the underside of the seat wedge. The velcro wraps are just long enough, there's room to use two of them to distribute the weight better, the light is as close to properly aimed as I can hope for, and it will be somewhat protected from the rain.