A broker is an implementation detail; there's nothing stopping you from going peer-to-peer with $protocol and having publishers manage their own subscribers. Some sort of registry is still needed though; nslookupd in your case.

There is a bit of an impedance mismatch since now you have to go out-of-band to fetch information. Not saying it's a bad thing; that's essentially also the role of DNS and LDAP, among others, so it is a fairly common pattern.