HTTP push to Unix hosts
Take a look at Udpcast - might of might not be what you are looking for. Here is some guy's blog about using it.
Comet is unrelated to this, each client will still have its own connection. If you have control of the network you could use multicast to send it in one go. Or if you have control of the clients you could have them all forward it to each other to spread the load out from the first pc.