The inner part of my upper, right leg has what I as a lay person would describe as "joint" pain (rather than muscle soreness, which I know what that feels like). It became noticeable the last 10 miles of a 62 mile ride last Saturday. I felt it a bit today and quit after 10 miles. I feel it when I stand and lift the right leg up, like it does on the up-pedalstroke.

Here's some data. My legs are built freaky IMO. Hanging freely, my toes point outward substantially -- I'm guessing 30 degrees. So I deliberately rotated my cleat a little a few hundred miles ago, probably not the full "30 degrees", cause it would look really odd/wrong.

I also have flat feet, and I have not been using my custom arch supports/orthotics. I put them back in today, with no improvement.

My left cleat is rotated just about as far, and my left leg feels fine.

My intuition makes me suspect that, even though my foot points outward, that rotating the cleat might nevertheless have caused the joint pain. Or maybe the feeling is that my leg is too far inward toward my bike.

Anyone have a hunch what this could be, without seeing me or my bike?

From what you've described I'm built exactly like you and get the same pain in my right inner thigh. I've also rotated my cleats out like you because of my feet pointing outwards. Weird.

Do you find that cleat rotation solves the prob? I feel like the painful part of my thigh wants my foot to be more parallel, and not turned out, but that's counterintuitive.

Well actually yea. I'm not sure if I explained it right but I rotated my cleats so that my foot points more inwards. Before I did this I would always get the pain. Since I rotated my cleats I only get the pain if I'm in the drops for more than a few minutes. I think the more my feet point INWARDS, the better it is on the painful part. It seems weird to push them the opposite way they want to go but I guess it's working so what the hey.