You could try hacking into wp-includes/js/tinymce/plugins/wplink/js/wplink.js
and change:
if(!i.href||i.href=="http://")
(two times)b.url.val("http://
You could try hacking into wp-includes/js/tinymce/plugins/wplink/js/wplink.js
and change:
if(!i.href||i.href=="http://")
(two times)b.url.val("http://