Thanks very much, I’ll look into the Set Master Pose Component action. 
To confirm, though, I don’t immediately see a way to “attach” one rig to another (as sort of a “sub-rig”). Does this mean that the character’s rig must be able to represent all possible modular configurations, and then the parts of the rig that don’t have a modular part attached to them simply don’t do anything?? If I’m understanding correctly, the various modular parts will each map to only part of the rig, and the Set Master Pose Component action allows them to be combined to create a sort of “compound” mesh associated with that one skeleton?