Has any player taken advantage of this rule in the last 50 years?
There are probably other situations where it would come in handy, though a kneel-down or hook slide is usually sufficient to indicate that a runner is giving himself up. The latter probably would have worked best for Bradshaw if he had thought of it.
On the subject of palpably unfair acts, has one ever been called in the NFL? The Wikipedia article cites a 1971 issue of the St. Joseph Gazette which states that the rule had never been used at that time. I can think of one game where actions by people off the field clearly affected the outcome, and it wasn’t invoked then.
Note that the pass also has to make it back to the line of scrimmage, though this wouldn’t likely be an issue unless the QB managed to run back really far.