now that i've started digging into Servus, it turns out that you cannot discover the port of a service.

e.g. in avahi.h, the uint16_t field which denotes the service port is simply unused:

now i'm not an expert when it comes to ZeroConf, but in my understanding the port information is crucial for service discovery (at least this is what i gather than you always must provide a port when announcing a service; also, i don't know how one would actually reach a service without knowing both the host-address and the port).

have i been misusing Zeroconf, or is this just a weird omittance?

I'd suggest using the servus_port key for this information (though i admit i wonder why Servus would reserve any key (e.g. servus_host) for private use for obligatory information (like host-address, host-port, service-name and service-type))