You won't do any permanent harm to your rabbit putting it on it's back although the rabbit may go into what seems like it's being paralyzed but in reality it's not really paralyzed.
But you should really not put the rabbit on it's back anyway because it might do something trying to get back on it's feet and harm itself.
Laying the rabbit on it's back may also cause it some pain so that's another reason to not lay the rabbit on it's back.