I agree you can do it, but in any company manuals I have seen it is forbidden. Also a good chance of FOD damage/ingestion. Aint that also why it is not recommended to use reverse at low speeds? Also as pointed out I don't think the airport would be pleased about you doing it. I have never seen or heard of a B757 powering back, not saying it hasn't been done though!
Just checked the Boeing B757/767 Flight Crew Training Manual:
Backing with Reverse Thrust. (Page 2.2)
Backing with reverse thrust is not recommended and may be prohibited by the AFM.
Eff Oh.