Would the lighting to 30-pin adaptor work on the iPhone 4 dock or is that too unstable?
I would suspect it would be dangerously unstable. The existing 4/4S dock uses not only the 30 pin connector to hold the phone, but the dock itself cup around the bottom of the phone to support it. I think using the adaptor on the dock would leave the iPhone 5 hanging in mid-air and it would likely fall backwards off it and perhaps damage the Lightning adaptor.