On the early odyssey freecoasters there was a hole drilled through the axle and threaded, I've put a bolt through it and ground it down so its fits snugly and doesnt interfere with the mech, what it does is stop the springs moving out of postion and acts as a spring tensioner. This cuts down on slack and it has not slipped since it was put in 8 mounths ago.
I was testing another mod from a bike shop that does something similar but doesn't need a hole in the axle, its being refined so watch this space.