If I remember correctly, ncp only works on LANs (using local segment broadcast), so it doesn't require either side to have any Internet connectivity at all. Your approach would be a lot more flexible, but I bet people would also appreciate it if you could figure out a LAN-only mode -- for better privacy, and when the Internet connection is down. Maybe you could start with an ncp-style segment broadcast and then fall back to the transit relay server if nothing on the segment answers and begins negotiation?
Yeah, also the two sides can exchange (encrypted) notes about their apparent network environment (including their public IP address, as reported by the rendezvous server), which might give them a clue that they're on the same LAN.
Also see https://github.com/warner/magic-wormhole/issues/48 for some discussion about making things work when you're connected to each other but not to the internet, e.g. an ad-hoc wifi network. It doesn't work at all now, but it totally should.
