I've used this benefit many times, but only in Coach for short flights. The SDF - DTW 5pm flight is often sold out. Each time it has involved hearing no, then pushing. Typically I get "Well I guess I'll ask a supervisor but they will say no" type reply - only to hear how shocked the agent was that it was OK to oversell.

I doubt this benefit is used much so the folks on the front line are not aware

Keep trying is my advice