The FS guys I've been on trail maintenance with have a mount that goes over the rear. Kind a modded rear rack. Worked great.
My buddies have tried several locations and said that the rear is the worst possible spot if you are trying to put any miles in while doing your trail work. It really changes the way the rear end bounces around. In the front you won't notice much added weight and it also seems to get snagged on less stuff. I have also seen the rear mounts and while they look very good and functional they seem to change the bikes ergos too much for a long and successful trial work day. Most of us have our saws in a backpack which sucks and that is why I want a mount like some of these that go in the front.
Now if they could just find a way to mount it and still have your bike be legal with the lights. I guess that is the benefit of the rear rack.
Make an led conversion like palmer did and either mount it on the side of the saw or above it.
Must be DOT!
At some point you just have to F it and ride anyhow. Besides what are the chances of finding Mr Law on those trails anyhow?
On another note, I've been told that "everything must be DOT legal" line before and it is only partially true. No one has written me a ticket yet and if I eventually get one, I'll just pay.