FastTrack does this (except it has a central server to fall back on if the client can't connect to any host in its list). You have to have a way to connect clients even if they have an outdated hosts list. And it would be nice if a client could connect without having to rely on any static node. You could do this using a single multicast address containing all the current on-the-fly hostcaches. But a problem is that if an ISP blocks that address it becomes much more difficult for some users to connect. Maybe some combination of a multicast address and a hosts list would work decently?