Wendy McMaster did this in a Canadian contest once. It was absolutely epic.
Heres the Wendy McMaster clip: https://youtu.be/jW5CWLvpE2g
BTW, in figure you don't technically flex on stage, but I loved how Christine Pomponio's pecs would stretch and pulsate on her own during her contests. Most figure competitors don't do that.
Some fans like the idea, but IFBB/NPC judges might frown on a contestant who's showing off like that. That's why it's rare. Flexing pecs is a different matter.
There's a lot I can say about that but short story...I think it's BS how women can't show off their physiques as much as the guys do but that's another can of worms for a different day and post.
But I dig any woman who can sneak in a subtle pec pop on stage, even if it's not a big one.
Have any female bodybuilders ever done a pec bounce on stage at an actual bodybuilding competition as part of her performance?