No, there's no markdown here to allow underlining and the CSS method isn't possible either because it's requires restricted tags or attributes.
You could use unicode. It has so much space for characters in 16-bits that they decided to do the whole alphabet with underlining:
T̲h̲i̲s̲ ̲i̲s̲ ̲j̲u̲s̲t̲ ̲a̲n̲ ̲e̲x̲a̲m̲p̲l̲e̲,̲ ̲i̲t̲ ̲i̲s̲n̲'̲t̲ ̲p̲e̲r̲f̲e̲c̲t̲.̲
Of course these mappings aren't simple to remember (I think there is a Compose-key layout for them but I forget the combination) so there are tools like Unicode Underliner that do the bulk of the work for you.
I would beg you to use it only when you really, really need it though. The last thing I'd like to see here is an all-out formatting war.