The OP refers to a situation where the new CP user tries to initally load an https URL (for instance, https://google.com as opposed to http://google.com)
You could redirect his initial connection to e.g. https://google.com to your own https server ("impersonating" google.com in order to further redirect him to your CP login page) but unless the user's browser has loaded your CAcert, it would result into various scary-looking warnings by his browser about "problems with the security certificate" recommending to him to close the page.