I'm reading about sound sets and I understand a lot of it. But let's say my sampler engine is loaded in Reaper at a particular MIDI port (I use Midi Pipe as a virtual cable). Where do I specify the port? Can I specify it as a name or must it be a number? Is it specified in the def-score function call?

Yes in the DEF-SCORE.

Evaluate first (midi-destinations)

to see how Opusmodus see your ports numbers and name

and after add into your DEF-SCORE instrument:

 :port n where n is the port number,

or :port “name of the port”

 

sb.