Specter Desktop implemented QR-based message signing a while ago. They defined a very simple QR format:
https://twitter.com/KeithMukai/status/1583932302115057664
My understanding is that nostr:npub1hea99yd4xt5tjx8jmjvpfz2g5v7nurdqw7ydwst0ww6vw520prnq6fg9v2 had some misgivings about the format but did implement support for it, too.
Then once it's signed, the returned QR is just the signature itself encoded into a trivial text QR.