I thought about doing that on the truck I use for plowing but I only do 3 driveways when it snows, so the cost isn't really justifiable. You could always mount something like a TIR3 on the side of the bumper, too.
As someone who has spent more then a few hours plowing I will say this; if you are going to he mag mounting this be careful about the bar you get if you do go halogen. Some of those bars will drive you crazy, it is like a meat grinder going on the roof. If you have to go halogen get a belt driven bar with somekind of rubber covered mounting system (or better yet do a BackRack or something). As for LED bars, my company has almost all LED bars on our one ton and under trucks now and have no problems with snow and ice build up. To me it seems that if it are clear when you start, they don't have build up. I would say do the LED bar for sure.
Mounted TIR3s in my reverse lights facing the rear.
I agree with going rotator, Star warning makes some inexpensive bars, extremely durable rotators (ball bearing rotator shafts, shock absorber mounted rotator assembly to reduct vibrating, prolonging bulb life) But my personal favorite is the old turbo beams, 16 square inch reflectors! I think theyre the biggest yet! I run an 8 strobe turbo flash, same 16 square inch reflectors, 8 bright (they do warm up) strobes.
I have found no snow/ice build up, even in the worst storms, with the L31 beacon. This was an initial concern when buying the L31, but did not manifest itself. There seems to be enough heat generated to eliminate that problem, especially if you start with a clean light. I have also used numerous halogen rotators, and while they give a superior warning signal, I, and many other plow operators find that the beams rotating and flashing off of the falling and fallen snow tend to be very distracting, especially at night. The more, and the faster the rotators, the greater the distraction. I have found myself turning off the rotators, defeating the purpose of having a light, and other plow operators opting for no light, due to this distraction.