Back to the original question, as others have noted, OSHA regulates scissors lifts as scaffolds, and has separate regulations on boom lifts. OSHA's view is that one can easily be catapulted out of a boom lift (although Mythbusters showed that that's pretty hard to do). That won't happen in scissors lifts.
Some GC's and owners require harnesses on those in scissors lifts in case the workers are inclined to climb on the railings. And, most scissors lift manufacturers (check their manual) recommend that users use harnesses. BUT the harnesses can't be connected to the railing. OSHA's scaffold standard forbids it, and the manufacturer usually provides attachment points--at pretty inconvenient locations.
Why not get to the root of the problem? The reason folks stand on the railings is that the scissors lift won't fit between the joists or mechanicals, and putting stepladders on scaffolds is forbidden, too. Maybe the solution is a smaller scissors lift or a trestle ladder.