No, that doesn't help at all, because it makes no differentiation between uncrossable as in "we think you might drown or hurt yourself" and uncrossable as in "if you do it you'll be disqualified".
If Ed is right, then that means I'm allowed to cross uncrossable marshes, crags, fences, pipelines, lakes and streams, it's just that the mapper doesn't think the average orienteer would be able to.
Obviously the event organiser could change the default position, by stating for example "fences must only be crossed at marked crossing points", as could the planner by marking an uncrossable marsh as OOB.