This has also been discussed before in Gitter. There is also some recent discussion in the Ironbelly group about sending via QR codes: https://t.me/ironbelly/942
The initial drawback was that QR codes could not support enough data to send large Txs. However, it’s been suggested that color QR codes could be sufficient and animated QR codes(fountain codes) can support large data transfer( much larger than any Grin tx) https://github.com/divan/txqr
Sending via NFC for in-person TXs was also another suggestion.