This app.conf setting only works for displays:
Default bundle display templates for related bundles (Eg. ca_entities, ca_occurrences, etc.)
Could there also be an option to set it for all relationship bundles for a record type, like related objects in exhibition records and related objects in loan records could use this single object setting?
<table>_default_bundle_display_template sets the default template only for relationships in displays. I've just added <table>_default_editor_display_template options to allow setting of default templates in editor relationship bundles. The two should be separate settings as they're often somewhat different.