Using a foam roller is a great way to elevate your body enough to decrease the amount of hip external rotation required from pigeon pose. I personally like a foam roller more than yoga blocks because it can support the entirety of your leg, not just under your butt. Now, you may be lacking this movement because you're tight, or you because of a true structural issue (torsion or the like). Either way, the goal with all mobility work should be to convince the nervous system, not try to beat it into submission.