About half way done with implementing pairing. The webapp's interface to prompt for a secret and start pairing is done; the protocol is implemented; broadcasting of pairing requests is working; added Yet Another Thread to listen for incoming pairing traffic.
Very happy with how this came together; starting with defining the protocol with data types let me rapidly iterate until I had designed a simple, clean, robust protocol. The implementation works well too; it's even possible to start pairing, and only then bring up the network interface to the machine you intended to pair with, and it'll detect the new interface and start sending requests to it.
Next, I need to make alerts have a button that performs a stored IO action. So that the incoming pair request alert can have a button to respond to the pair request. And then I need to write the code to actually perform the pairing, including ssh key setup.