The way I set up tags for my documents in OmniOutliner is to create a “Tags” column and simple enter keywords there. It’s up to you whether to prefix with @, #, or nothing, comma or space separate and easy to use for automation. Step 1 might be to implement something similar (multiple column support), Step 2 to have column types including a special tags type that follows standard macOS tag functionality. See also: Future Feature Request: Complex OPML handling