Yes currently this is not an option, I am changing this to an Idea so that it is considered for a future update.
You could run a Registration Report to get the Email addresses of those who were denied, and then send a personal Email to the group, maybe even include a registration link for a repeat of the webinar.
When you currently "Deny" a person for attending it sends out a generic and very unfriendly email... I've literately had harsh responses when sending it out, not good. Similar to the reminders, invite, etc. allow the organizer to compose the subject and body of the denial.
Today, I have gone to not denying folk but rather downloading the list and sending them nice well thought out rejections. But this is a huge killer of time.