Is there a way to prevent uke from rolling out of a shihonage pin? How did shihonage become a pinning technique anyway? Maybe it isn't meant to be a pinning technique?
I find two opportunities for escape in shihonage. First after shite/tori has shifted direction and is attempting to lock uke before throwing. The second is after the throw while on the ground at the pin.
I believe that you refer to the latter.
I find that what is important is that you setup the pin while uke is being thrown. That is, you need to extend your wrist so that the wrist and forearm are straight rather then have your wrist back. and you need to do this before uke is on the ground.
When uke is on the ground, with your wrist straight inline with your forearm, her elbow should be extended enough over her head that it is close to the mat. If her elbow is up hight and free next to her head she will be comfortable and happy and able to escape. With her elbow extended it will lock her shoulder better and somewhat suck in comfort, and she will be less inclined to attempt an escape. At the same time, your leg that is close to her should tight up against her side to prevent her from rolling. (If uke is really really flexible, forget the pin and atemi to the head.)
hope that makes sense. In general i dont attempt explaining techniques in email any more... its too hard to explain.