What I am planning on doing is to use one of the 'backpackers' headlamps for this. While I have not yet done the final step and taken a test flight, the lamp I am going to use is a "Black Diamond ~ Gizmo" A link is http://www.rei.com/product/825732/black ... o-headlamp
...it's now on sale for $15. I will use a strip of velcro on the Phantom and a mating piece on the "Gizmo" after removing the supplied head strap...that seems simple enough..! The Gizmo can be operated either as a steady light or it can be put into a flashing, sort of "strobe" mode. In this mode, it produces pretty bright flashes about 2 times a second I will attach it to the rear of the Phantom so as to not interfere with the camera end. I'm not sure how far away this light can be seen, but I am pretty certain that it would exceed my "Comfort Distance"...! It has a pivoting mount so that it can be aimed in a downward direction for better visibility.
I have mine here and just weighed it, both with Alkaline batteries and the lighter Lithium cells.
With Alkaline ~ 1.6 oz
With Lithium ~ 1.3 oz = 37 g
That's my two cents worth....