Based on #2803875: Node form meta information should not come from a theme and #2882801: Review and improve the media creation form, I had the idea that we could let entity forms/modules opt-in to the seven sidebar by setting a flag on the form structure, for example $form['#sidebar'] = TRUE. possibly also on the specific elements that should be in the sidebar, although that could maybe be done through the existing group functionality as well.
Not sure yet about all the details, as the current implementation relies on a specific template that seven activates for node but is actually defined in node.module (yep, weird) and it does not have columns in node, but in *classy*.
So maybe instead of introducing a flag, we could just introduce a content-entity-edit-form template that seven can override and make it two-column? Not sure about BC for node, though.