Underline is currently handled entirely by the stylesheet. However, you can bake logic into the template to look for a specific role (such as underline) and use the <u> element instead.
In the inline_quoted.html.slim, you'd use something like:
- case type
when :emphasis
em class=role =text
/...other built-in types...
- else
- if has_role? 'underline'
u=text
- elseif role?
span role=role =text
- else
=text
You can tweak the logic as it works best for you.