> unfortunately an external STUN server is still required.

STUN is only required for NAT traversal. Clients can generate SDP offers without STUN, TURN, and ICE, i.e. offline; but an offer is still useless without some means of signaling.

> Would be great if the browser gave us a way to discover peers fully in-browser

WebRTC never specified signaling so this is a tough challenge. Just my two cents, a first-class SIP API would go a long way towards normalizing signaling.