We have a potentially infinite set of signals and data we'd like to communicate to programs
Ah, I think I've identified your problem.
Signals are for alerting programs to a specific (small) set of changes of external states. There's nothing "potentially infinite" there (frankly having 2 different SIGUSR's is generous).
For transmitting arbitrary messages, Linux provides message queues, which are very nearly sockets (and the ways they aren't sockets make using them for IPC easier). What's the limitation there?
It lacks multicast, sure, but then receivers can just make their own queues and have the broadcaster send to them if you want that.