This is a good one, and I do this when I do the sled (which for me is a heavy tire with a rope that I pull instead of push, but it feels about the same on the legs).
actually you need internal rotation to squat, especially deep. Search this article: ( http://www.T-Nation.com/free_online_article/most_recent/squat_like_you_mean_it_tips_for_a_deeper_squat )
They've basically said "don't squat with weights" and haven't been helpful in programming other than telling me what I can't do. I haven't tried many more or seen any trainers because, well, I'm poor and they charge too much. But reading on my condition (Hip internal rotation deficit (HIRD), Femoral-acetabular impingement (FAI)), there's basically nothing I can do to fix it. I just have to work around it. I have a normal range of motion but it's shifted towards the external rotation, meaning I have bonus external rotation but an internal rotation deficit.
I've wondered about this, but I'm hesitant to dump it until it starts hurting. It doesn't seem to put me in as much of a compromising situation. It's more hip flexion + internal rotation or extreme hip flexion, as in, all the way. If I keep my hips neutrally rotated, I can flex enough for a deadlift with no pain. Of course, no pain doesn't mean no damage, necessarily.