It is not clearly or officially recorded about such an attempt however the daredevil might be seriously injured. There is usually a limit to everything and dong a back flip at such extreme height is highly dangerous. There are people who with enough and adequate practice over a long period of time attempt such things and become successful. Find out from an expert about the dangers involved in such a move.
I'm sure it's been done many times. Visit your local Emergency Room and ask how many broken legs, fractured tailbones, and disarticulated spines they've worked on as the result of people doing back-flips off bridges. As they say on TV, don't try this at home.