You should be able to underline a space, but why not use a real rule below? You can set that to column width with any indents you want, regardless of the characters in the line.
Thanks, Peter, that's a good option. Because some of the equations are odd, I wanted to have an easy option for adjusting the gap between the divison bar and the numberator and/or denominator text. I was (or course) hoping for a magical 'option+command+control+shift+whatever' stroke as a fix.
For anyone who might read this string with the same issue, there is also the option of placing the rule above the denominator text (rather than below the numerator text), thus making the divison bar span the larger text line.
you could use any character (besides space) and give it a fill of none
... as long as the underline/rule is not [text color]