With some scripting I think you could subvert the existing added_at field for this purpose. You could manipulate the database, joining metadata_items ↔ media_items ↔ media_parts, updating metadata_items.added_at to match media_parts.created_at (or updated_at).
Or, and probably cleaner, your automation tools could trigger a script when items are refreshed/upgraded, which could add those items to a “Fresh!” collection in Plex.
