I did a variation of that for my onlineforextradingtoday.info blog,
basically I followed the instructions from this post(last comment on it) http://ww.wp.xz.cn/support/topic/commenting-on-draft-posts?replies=2 .
Basically what it suggested was similar to yours except rather than commenting it out changing the comment_on_draft to the following:
do_action('pre_comment_on_post', $comment_post_ID); and then removing the exit; right after it.
