While i understand that it is largely expected to be driven by the API, i expected to be able to also expose a form on content entity.
Document how this works.
Turns out my main problem was not seeing how to connect it to specific content types in the first place, once i realized i had to dive into the mysterious "Other (revisions): An entity field containing an entity reference to a specific revision." option in the Add field interface:
i was able to edit the widget of first the Entity Meta Relation field and then the Entity Meta field on that in order to show my well-nested monstrosity:
Because a common use case is indeed showing it on the form, and i don't have the use case of needing to update an entity's metadata without touching the entity, i think i'll only be using ECK + Entity Reference Revisions + IEF Complex Open Widget to put things more directly on the entity with a quicker-to-good-enough UI. But putting this here in the hopes it helps others / stimulates more documentation of these cool modules!
Active
1.4
User interface
Just to note that Entity Construction Kit and Entity Reference Revisions do not play well together yet, so if you have a similar use case as mine you should probably stick with Entity Meta Relation (i fell back to three fields in a field group repeated across a few entities).