Well I can only speak for Panther and con-way now( before the buy-out), but I have always been able to book my own loads when they had nothing for me to run or I had a directional need, the only thing I have had to do is call the dispatcher or with Panther call the backhaul dept, and if the broker is not on the "do not use" list, they have always been happy to set the load up. As of right now, I am on a backhaul out of Ft. Bragg, N.C area heading to Ohio, with a quick drop in Pa, that is paying as good as a regular load for panther.
Panther gave us a list of brokers not to use, and set up a couple guidlines for us and told us when we needed to backhaul, set it up. I average about 1 broker load every 8-10 days to keep my truck moving.
Bug I dont know if my post helped answer your questions, but that is what I do.