Include custom_excerpt as an option in newsletters

I’d really like to have the option to include the custom_excerpt in my newsletter (with a toggle switch like the title and featured image would be perfect).

I use Ruby, which uses the custom excerpt like a subtitle (seen here: Can I turn off making users confirm their Ghost subscriptions? )

It looks great on the website, but when I email the newsletter, the excerpt doesn’t appear at all. I could put that text at the top of the body text (or I guess in an email-only card), but it’d be great to be able to get it above the fold in the email viewer!