Hm! good point...
I think part of it has been solved by the recent changes in T210099.
Namely there was a bug in accessing capsule fields that had underscores in them, like geocoded_data. This is solved.
However, there's still some additions needed:

To add a dimension on a subfield of a struct field you can do now:

event.namespace_id

And the code will "flatten" it to event_namespace_id (and do all this implies in the ingestion spec).
To do the same with a map field, you would do:

geocoded_data['country']

But the code is not yet able to flatten that syntax into geocoded_data_country.

So we should change that. However, I think it will be easier now.