i know what you're saying, but you've just described a finisher rather than a back off set.
a 'back off' set is done as a last set of an exercise once you've completed all the heavy work. the idea is to use a less weight, with higher reps to target the muscle fibres in a different way using the same exercise still.
what you're talking about DOES have a lot of merit, i just think that the best way to improve on the exercise is to do it more (sensibly). if blood flow / glycogen recovery is your purpose for doing it then thats ok, but i'm just saying it makes more sense to use the main exercise.
In a nutshell...
Bench Press > Chest Fly 'Back Off' - you will get a pump to serve the purpose you described.
Bench Press > Bench Press Back Off -you get a pump, more volume on the main exercise which will have a better stimulus effect on your overall look.
good luck to you whatever you decide sir!