Doug Larson shows one of my favorite shoulder stretches.
I like to do this one after extended periods of sitting. Great to improve your posture, but of course also useful for your overhead position.
Variation: After Pressing sessions I like to bring the intensity of this stretch up to 11 by using a medicine ball (gives a greater ROM) instead of a foam roller and holding a 5kg in each hand (roughly 3x 30-45s).
Check out Mobility 101 for more stretching ideas.
Also check out the Zygmunt Smalcerz equivalent of this stretch.
Update: Here is Apti Aukhadov doing basically the same thing with one of his seminar attendees.
I wonder if this alone would be effective enough to gain mobility necessary for the squat jerk.
Doubt it.
You need weight pushing down on you. Best mobility exercises are the exercises themselves.
Well, obviously you would have to practice the squat jerk itself for a long time to gain real mobility in that position. But for lifters like me who barely have enough mobility to do a decent power jerk, this seems like a good exercise to gain the initial mobility to just squat jerk an empty barbell.