We have a situation where the

This sounds like it could either be a very successful approach if each contractor is committed to doing a good job and taking responsibility for their piece. On the other hand if they don't understand the requirements all that well or don't follow them closely, it could turn out not so well. To document it, you'd need to decide whether to track waste by weight or volume, and establish some system for documenting the waste that leaves the jobsite with each sub, and what they do with it. Does the GC have any documentation of his past success in terms of diversion rates for the project?

I think you are safe in terms of meeting the credit intent and requirements, as long as you can find a way to document it effectively and to make sure that the waste is actually diverted.