I wanted to do that on my touring bike but couldn't. I mounted the light on the front of the front rack.

#04b Miele Uno L.S. Rebuilt 4 Touring - Added Bottle Mount, Canti-brake Studs & Bridge by Miele Man, on Flickr

It could also be mounted on the loop of a front low rider rack. i used a P-clip then.

You're correct that the lower beam angle gives a lot more shadow highlighting.

Cheers