Default excerpt for parent of a custom post type

I’ve created a hierarchical post type called mycustomposttype. Parents are created in the backend, children via gravityforms.

Due to my theme setup, I would like all parents in mycustomposttype to display the shortcode [mycustomshortcode] in the excerpt.

To start, I’ve tried to auto-populate the excerpt for all mycustomposttype posts.

This does not produce any excerpt when creating or editing posts.