Not sure if it would cause damage, but I personally wouldn't do that.
Would feel uneasy about the instability and the existing probability of some sort of break or damage. Could potentially break the Lightning connector over a long period of time and harm the internal port.
For now, try to pile up some small blocks of some sort behind that iPhone to prop it up so there's no force going backwards, only downwards.
Also, like the previous post said, I would be afraid of an inadvertent bump possibly from trying to grab the phone off the dock.