You write the note on your desktop and scan a QR code with its contents. The mobile app signs and publishes it directly to a list of relays it has internally.
Discussion
So, we are back to a list of relays? :)
Nice catch, yes, that came to my mind too. But writing to is much simpler than reading from -- still this app could allow you to pick the relays to publish each note to in case it's a contextual thing.
Actually it makes more sense for the target relays to be exported inside the QR code too from the desktop app.