Ok, seems moving the vang from the mast base over to the rail to act as a preventer is generally discouraged but possible.
So instead of a seperate preventer, I think some of you like the idea of a boom brake. Is this an entirely seperate device from the vang? Is a a stretchy line that one attaches to the end of the boom, runs forward to a block and then back to a cockpit cleat? And the brake is somekind of friction device somewhere on this line? Where do I find one? Sounds like it would be cheaper than the block and tackle set up of a preventer.