Tom, if you rigged a preventer from the boom end forward (maybe the bow) and then back to the cockpit that would work. Running the lines outside or inside the lifelines is a pain -- and I would suspect you'd want to have the linse about even with the first stanchion(s) may not be so "simple".

Anyway, I see you challenge.

I wouldn't use the vang as a preventer unless you really are in pretty docile conditions. A broken boom is a real problem and danger in those sorts of wind and sea state.