Thank you for sharing. I'm having a little bit of difficult time imagining your recommendation in the last paragraph above. Is there a video online that kind of shows what you've written above? Thanks in advance!
Here is a video using a much smaller ball: https://www.youtube.com/watch?v=OQuQ_aKlDjI
Here is another one: https://www.youtube.com/watch?v=_F-0rtFbOgI @ 1:15 is about the position that I use. Lying down, one leg out to the side and one leg straight back.
I use a slam ball but a soccer ball or volleyball that is adequately inflated would probably work too. If you are tough and flexible you can use a softball or something much firmer but it will hurt a lot more if you are not ready for it.
I regularly roll out my ass and upper back with a lacrosse ball or softball, but haven't tried it on my psoas yet.