Oh man, this is a tough one. I have chronic knee discomfort due to knock knees, and I will say that this work does help heal and strengthen. The best guess I have is any of the floor abs, where you are strengthening your core, which includes your hips, without added weight on the knee or standing. Following your own idea where you might strengthen that hip flexor as an experiment, since that seems like a safe place to start!
Replied on Hi friends.. I completed M/<45 FUL...
28 May 14:28
Appreciate a more restful approach. It reminds me of the Bruce Lee quote "You do not find inner peace. You choose it.."