Adding encryption shouldn't be hard, as long as you can communicate the keys out of band.
Could probably achieve the same result using audio instead of video, but maybe that would be too annoying?
https://en.wikipedia.org/wiki/Timex_Datalink
It'd transfer phone numbers and calendar entries to the watch by blinking the screen of your computer. Kinda a 1 pixel version of this.
(I wonder where that ended up? From memory I threw it in a drawer when I finished the Windows 95 project I'd been working on, and switched to Mac. There was no Mac software to talk to it.)
If your firmware is small enough to fit that model, it isn't a bad variant. Beats downloading a file, transferring it to a USB drive, sticking that in,...
Bringing it into the ultra sonic band would help.
It's interesting that you learn to recognise patterns in the burbling tones after a while. "CQ CQ CQ" is very distinctive and after a while you can hear your own callsign in it. I guess it's not that different to very fast Morse.