Hi Chris!
I’m afraid we can’t offer guidance in relation to custom coding with Follow-up Emails, as that isn’t our plugin.
Extracting the custom field value (if you are using “regular” WordPress custom fields) ought to be easy enough, though, by using get_post_meta() — the use of which would be something like:
get_post_meta( $event_id, ‘_your_custom_field_name’, true );
Good luck with the project — and if you discover anything useful that might help other FUE users it would be great to hear back from you 🙂