It’s still on our todo list to support this directly, but I wanted to mention that it’s currently possible to password protect a note in a roundabout way: by importing a password protected PDF into the note. This will password protect the note with the password of the PDF. You can even delete the PDF from the note and the password will remain on the note.Another option commented
For now I've protected the Papyrus app using Smart AppLock (SpSoft). Another option is AppLock (DoMobile Lab).