Somewhere along the line, the selection mechanism for fields to override stopped working to show ordinary Drupal fields.
Add an "Entity reference w/custom text" field, choosing to reference a content type you know has lots of plain text or rich text fields.
On the Manage Form Display page, for this field you created, choose Rendered entity for the Format. Then, press the gear icon to configure and choose the View mode and the field for which to Use custom text to override.
Note that none of the fields you know the content type has are showing up as options.
Needs work
2.0
Code