Afaik you can use items or things configured in Paper UI in your .items, .rules or .sitemap, but you can’t use items configured in .items in the Paper UI.
Be aware that Paper UI is not meant to use it as a UI for daily use, this is what you would do with Classic UI, Basic UI or any other UI (except Paper UI ;)), because Paper UI is only for configuration (control panel is more or less to “test” if things and channels are working correct). Paper UI is for administrators only.
So, this “restriction” is not a severe one 