We could support a reliable multicast protocol and use that to distribute messages across. This page braindumps how that could fit in with ActiveMQ 4.x's architecture.
So the brokers communicate with a kind of Bridge which uses the multicast/UDP transports underneath.
Each broker will repeatedly send its own BrokerInfo to the cloud. The MulticastBridge will receive them and keep that broker alive; brokers which don't send the brokerInfo within the heartbeat window are logically considered to have closed.