I finally have credentials to get into the Metrc sandbox! Unfortunately, I don't have any documentation other than the this work in progress: http://media.wix.com/ugd/73c73e_e930c713c46f4c08b07e7c2e2d29a2b5.pdf
I've been able to retrieve rooms, plants, items, etc., but that's it. While I can report waste, without any other documentation, I'm left to guess at the endpoints and payload required for other calls.
I'm waiting to see if I can get my hands on better docs. Until then, I can't do much more.