Ivan Ivankov at the 2005 Worlds
I have a very good athlete with a similar problem to yours; although not nearly as extreme. The following is a new stretch that I learned at our most recent National Team Camp. It will require the assistance of a partner.
1) Lie prone on your back. Extend your right arm up into the air.
2) Have a partner kneel next to your right side and, using their right hand, push your scapula (shoulder blade)very firmly both inward and downward into your side so that no movement of the scapula is possible.
3) While maintaining pressure on the scapula, have them apply downward pressure to your right arm. This will result in the arm moving back behind your head to the floor. I have found that it is best to apply the pressure above the elbow, closer to the triceps.
4) It may take a few attempts to get the hang of this stretch. Given your extremely tight lat insertion, only a small range of motion should be possible. If your arm goes easily down to the floor, probably insufficient downward pressure was maintained on the scapula. In that instance, simply reset and begin again.
I have found this to be a very beneficial movement and use it with that particular athlete on an almost daily basis.
Yours in Fitness,