I haven't tried it, but I've often thought it could work. The ditch has the shape (in most places) of an inverted top hat. About 30' out, it drops from 10' deep or so to 60' and that's where the fish hang out. Maybe we could fish it sometime and you can test your theory.
I've also wanted to see a spey rod in action in the surf. Might be a better way to do it.
BTW, you don't happen to fish the Pilgrim Nuke plant by any chance? I met a guy originally from the UK fishing spey there two-three seasons ago, wondered if it might be you.