Customize "Add comment" form in admin

This is part of a larger plugin I am developing which has various custom post types. For one of the post types, commenting is very important but is only done via the admin area, not the front end. I want to add a checkbox in the "Add comment" form box in the admin which would give the person adding the comment the option to have an email sent to the author of the post with the comment text. I already have the actions set up to have the email sent, but right now it sends on every comment.

See screenshot:

I can't find any filters or anything in the WordPress core that would allow me to add this customization.