one of the popular sound streaming clients does some kind of local relay thing, but I'm not sure which one.
I agree, multicasting is a great idea and its a shame it has not been supported as well as it could have by backbone providers.
From an economic perspective, unicast allows backbone providers to
sell more bandwidth, and that's where they make their slice, so implementing multicast is not compatible with their business objectives.
One possibility would be for the government to define "base internet service" and have that include standards-based multicast routing, that is what it would take to counter the business objective incompatibility problem.
Another possibility is shadow full mesh VPN networks, but that's not really practical on a how-do-you-get-there-from-here level either.