Roughly 30 years ago, I was on a lot of certification and engineering flight testing on a 767/CF6-80C2.
After one of the flight tests, we went to a remote corner of the airport and used the reversers to back the aircraft. When I asked why we were doing the test I was told it was to make sure it would work - so if an operator came in and wanted to use the T/Rs to back the aircraft we could give them a "No Technical Objection" (NTO). It was intended to cover the exact sort of circumstances as this thread - urgent need to back up and no tug available...