Wikibase provides data access to entities via parser functions and Lua functions. That access mechanism needs to work to access MediaInfo entities stored file description pages using MCR. That is, the wikitext slot has to be able to access the MediaInfo entity stored in the mediainfo slot of the same page.
Note:
* this is not a requirement for the MVP rollout, since that only covers multilingual captions, not full functionality for statements.
* this should "just work", since data access is implemented via an EntityLookup, and no knowledge about SlotRecords or Content objects is needed outside the Entity(Revision)Lookup implementation (see T194729).
* since there is no plan to support atomic edits across the wikitext and mediainfo slots, rendering of the wikitext can always be based on the stored version of the entity. So no access mechanism is needed for unsaved data, which would be needed for full cross-slot transclusion support (T187935).
* however, the wikitext (main) slot needs to be re-rendered when structured data changes (subject to usage tracking), which in turn may be tricky because of replication lag: if the re-rendering is triggered before the data is available on all replicas, the old version of the data will be used for the new rendering. This needs to be avoided somehow.