I think you could do all of this through Twitter. You could setup multiple accounts for each message type - let's call them broadcasts. Clients would follow the broadcasts they're interested in, and (most importantly) you can setup each twitter client to popup notifications for only the broadcast(s) they're interested in. You could use Tweetdeck to send from different Twitter accounts.

I know people who do this for swim meets - the officials tweet about which heat is up next so that the parents can approximate when their kids need to move from the lawn to the pool.