I have kids who come out with linebacker shoulders. Two things I have learned from that are:
1) My body was pretty clear in my unmedicated birth what the best position to give birth in was. Any other position caused me tremendous pain! Once I got onto hands and knees, I felt the strong desire to PUSH and he was born rather quickly. It still took some effort, but then, he wasn't a tiny thing.
2) Keeping baby in a good position was key. I did that by walking, rocking, swaying my hips, and NOT LAYING DOWN! My body again was really clear on this point. I tried to lay down when contractions were about 5 minutes apart so I could snooze between them (I had a really bad cold) but that was not happening. The pain while laying down had me up and moving again within one contraction.
I honestly believe in an unhindered birth, where mom can relax and really listen to her body without distraction, it won't be an issue. You can see on my video of Noah's birth when his shoulders came through my pelvis. You can actually see my hips pop out a good couple inches. It's sort of strange to watch!