Does anyone even use _ in place of * in *italic* or **bold**? If we can get a corpus of current markdown comments, I would like to know the amount of usage of _. I’m guessing not enough people is using it enough, and could be used for underlining purpose without much harm.
(I personally never used _, never seen the point. But I can see the usefulness of underlining or highlighting like _this_text_is_underlined_. )
I think if you want to implement stuff useful for reviewing, check out http://criticmarkup.com/ for inspiration