I've been looking though the source code for examples of this, but I'm stumped! When a Block is being edited, the URL changes to include

/modal/umb-modal-workspace/content/...

. Is there any way at this point to get the original document's unique/GUID?

UMB_DOCUMENT_WORKSPACE_CONTEXT

doesn't seem to be consumed at this point

Can you consume another context that may have the info you need?