I think we have something similar to this in MassMessage: a JSON content model that contains and displays displays wikitext, and a structured spamlist. (example: https://test.wikipedia.org/wiki/Yay_mm-ch!).

It seems that we want a "multi-part" content model should be either completely visible & explicit, which would mean it can't be edited directly using the "normal" EditPage, and reading and editing via the API would require the client to understand and process the multi-part wrapper data structure.

Or it should be completely transparent, exposing only the "main" (typically wikitext) content even when editing using EditPage, or even when asked for revision content via the API. Any "extra" parts would need to be requested explicitly. This means core could has to know about this special kind of content model, and implement special handling for it in several places.

I think it would be good to have both: a basic multi-part model, as well as a "transparent" multi-part model, built on top of the "normal" one.