One Squid feature that I find interesting for the purpose of sharing limited bandwidth is the ability, if I remember well, to slow down downloading of large files after a certain byte weight. With this the download of small files (html pages, little soft updates,...) can be done at the max speed, while large files (isos, ...) are slowed down after a while. This doesn't concern P2P, though. If your bandwidth is being burned by P2P users, Squid won't help !
I think ther must be a way with ISA (and, perhaps, DeleGate) to devote a fraction of the total available bandwidth to certain protocols, at least that's what I would look for. Then Squid can help me manage the http-devoted bandwidth to make most of 'em happy.
I don't know the specifics of http-encapsulated streaming (I'm no developer, sorry) but the principle is to use http to convey the audio/video stream, the receiving software is responsible for caching to simulate continuity. It differs from other streaming protocols which open different ports for stream control. Needless to say, the choice goes to the server operator, not the receiver, so if you can't block anything the only option at your disposal is to limit the bandwidth available to the different protocols, on the destination port basis.