From the “What’s the simplest thing that could possibly work” department… a link that creates a PM to the appropriate group is probably best.
e.g.: Upload your proof here
[Upload your proof here](https://meta.discourse.org/new-message?groupname=proofreview&title=My%20proof)
(this link won’t work for Meta since we have no proofreview group)
A theme component that added a button to a topic or post that created such a PM with a link back to the original post would probably suit well.