The safety in most double shotguns merely blocks the triggers from tripping the sears. I am assuming most grip safeties like the ones described here are the same, i.e., they only block the triggers, and do nothing more to keep the tumbler in bent. So it seems a similar arrangement on an exposed hammer gun wouldn't do much more to prevent an accidental discharge in the event of a fall, than on a cocked hammerless gun that doesn't have intercepting sears. It would only prevent the shooter from accidentally pulling the triggers. And even intercepting sears are no guarantee against an accidental discharge.
The stalking safeties we sometimes see on exposed hammer double rifles would be an exception since they mechanically block the hammers from dropping.
