Hi Thad,
Coincidentally I am drafting an abstract on exactly this topic. So yes I am keen on getting more of these properties in place. Bare mind that getting properties proposed can be challenging. I will draft the proposal for close match. Do you have a show case example, I could use to draft this proposal. An example where an actual Wikidata item, maps to the one or more systems with more granularity.
Regarding the external subclass, why not create an item for the external subclass in Wikidata and map that with the internal sub class property (P279) and then use the property exact match (P2888) to map the new item to the external class.
Cheers,
Andra