Private requests / how to block content without a new library

[pcm]

#2 -- The 'tags' feature would be perfect if there were a way to update those manually, but it seems these are assigned via TMDB or similar.

You can manually update the tags and check the 'Lock this item to prevent future changes' box. That way TMDB will not overwrite/update your tags. This is available in 'edit metadata' when you right click an item. 

#3 and #4 could be possible, but are these also pulled from an external source? No way to manually update a rating?

Again, these are available to be manually edited/changed if you right click and select 'edit metadata' on any item in your library.