I think you can't coach the release because what happens during followthrough is a reaction rather than an action. If someone is taught about correct finger placement, good alignment, reaching a good anchor point and employing the correct muscles, I think the release will take care of itself. Easier said than done of course.
To draw an analogy, you can't coach someone to make the bow react in a particular way after the shot, because it's a symptom rather than a cause. Same for the bow arm. If someone's bow arm drops like a stone you have to address the actual cause, not the symptom.
Hope this makes some sense.