I forwarded the question of how these were used to the 10th Mountain Division association in Denver. They don't have historical documents about it any more, but a member of their "Living History Display Group" responded with information. That information is posted below with his permission:
My role has been to work with the veterans to determine how equipment was used, as well as the development of that equipment.
The rifle attachment was as you describe. The hook on the frame was probably bend in post war use, as this hook was frequently used to hold canteens, climbing equipment, etc.
I am tall, yet have found that the muzzle of a M1 Garand hits the ground if deployed as the Army intended. The drill was to release the strap with the left hand, pushing the muzzle of the rifle to the rear. As the rifle fell, it pivoted at the butt swivel and was caught by the right hand extended down and to the rear. To clear the ground, the muzzle is swung wide and to the far right, and the weapon brought to the hip to be fired. It could be detached as needed for aimed fire, but rapid deployment was intended for firing from the hip. (Really!)
In practice, the soldier undid the strap, and the soldier's buddy helped unlimber the rifle or the pack was removed and the rifle detached once the pack was dismounted. As they were not used in combat in this manner the design was never revised.
In Viet Nam where these packs did see combat, the rifle strap was only employed to carry extra weapons like the M79, as your prime weapon was always to be in your hands.
I hope this helps.
I'm very pleased to see that there is someone who a) maintains this information, and b) is willing to share their expertise.
Looking at the dimensions, I'm fairly certain that this maneuver would put the muzzle into the mud for me as well but I'll try it with a helper.
