I am assuming you mean something like a martingale? I personally would not use one, especially not just to cover up the problem. The only place where martingales should be used in my experience is for a horse's balance at high speeds. (With that said, I would prefer a horse to learn to balance on its own prior to doing such events.) It is a device constantly misused to tie down a crazy horse's head, and if a horse relies entirely on it, without it they can flip over backwards. (I've seen it happen.)
And, as Taffy Clayton has pointed out, backing can be a very dangerous thing to teach a driving horse. We have a mare who went to training and is now collar shy, and will try to just back up to get away from the pressure. With horses like that it is hard to fix. There is no whacking this horse on the butt and expecting her not to take off like a bullet. Already not liking the pressure, I could only assume she would go as fast as she can. (She was pulling carts very well, then was moved into fieldwork and her problems worsened.)
I wouldn't see a problem with asking maybe one step, when we are ground driving and pulling a tire, often times they stop and the traces are pulled tight, so we have them shift their weight backwards so we can unhook them, but nothing more than that. As long as you have a very good stop, and good stand, you shouldn't need to teach backing until she has accepted most of what you can throw at her, and is in the cart driving willingly.