Oh, a LOT. Like, hours of gameplay. I discovered I had missed it right when I was acquiring Barry's Jump-In after his Side Quest 3, and he has been jumping in consistently. Also, on Maiko's character screen, there is no indication that she has the skill available, unlike on Tiki and Barry's pages.
I am hoping that my game just glitched and that I can get it in a New Game+ from this file. (I am at the final boss, and wanted to get this working before creating my Completed File, but if I have to, I guess I can go to that point in a NG+...)