Check out what I ran across when I was looking at clothing online, ladies:
Nothing Bare Pocket Hipster.
I admit that page doesn't do the best job of explaining what it is, so here's the original, sans pocket. You're meant to wear it under a shirt. Admittedly, you'd need to make a hole in the pocket, and I don't know if the pump would be too heavy, but I still think it's pretty cool!