Yes something like that has been done, the solution uses asymmetric cryptography instead of symmetric and it protects the entire communication stream instead of just opening the gates - it's called Wireguard.

I jest, but you're describing authentication protocol which uses the UDP destination port field to carry each character of the "one time password" in a roundabout way. You could simply send 1 packet containing the password to the daemon.

I've never seen it used in the wild but apparently, it exists: https://github.com/mrash/fwknop

That’s cute. Have Claude do it. There is already an echo system of TOPT QR to communicate it, so you would just need to convert your six digit code to two or more hits on the 16bit port range.

I’m not even sure you need more than two. One seems bad because that’s just a lucky port scanner one time.