What struck me about your post was the “temporary” bit. Not so much the “adding a tag” bit.
What if you used a meta/utility note that was just used to temporarily collect notes by linking to them. Like a Map Of Content basically. You could add notes to it (with a command or whatever). To remove the collection, delete that note.
And then the outgoing links from that meta note would be the way it referenced the notes, rather than editing the content of the notes themselves. (Editing the notes themselves would trigger syncing, versioning, modified date changes, etc. And you’d have to figure out how you would add the tag content. Prepend? Append? YAML?)
And then the key… it would be really useful to have a search operator for querying links. For example, say your temporary collection MOC note was named “Gerbils”.
And then be able to have a search like incoming: Gerbils or outgoing: Gerbils to filter that temporary network. And then you could also filter by saying -incoming: Gerbils or -outgoing: Gerbils. You could find all the notes that are linked from one note, but not linked from another note. All without editing anything in the target files.
Does anything like that exist yet?
I’m trying to figure out if Vantage plugin can do anything like that. It has some notation for finding links with a certain text in them. But I don’t think it can just query “all notes linked from X”.
edit: I found this related feature request to what I was describing: Search Operators for LINKS (backlinks, forwardlinks)