Hey,
About the low crawl, it's a bit more complicated, as the hands are also connected to the rifle. If we had elbow effectors, they would fight with the hand effectors, as there is only a single bone between them. You could instead just use the hand effectors, then run a code after IK, that makes a spherecast at the position of the elbows, and twists the arms so that the elbow ends up on the surface.
Anyway, it would be extremely difficult to add those effectors, I'm not sure the algorithms would even support that.
Best,
Pärtel