It will be too late. Users will have already seen the error message and had to accept it by the time the redirect happens unless you get another certificate. The previous answer has that problem.
Ignoring that, you can create a named based virtual host on port 443, set up ssl on that, and then have it rewrite. Note that instead of using your existing certificate (which will generate an error) you can have a cheap, valid certificate under mysite.com on that virtual host so that you don't get an error prior to the redirect.