> The only hitch I've encountered is that if the sentence spacing is > greater than the groff default, the underline gets broken between > sentences.
Yes, also if you switch to, e.g., bold while in underline mode. In fact, in anything which isn't printed with a single "show" (or one of its variants) the underline gets broken. I think it works surprisingly well for being such a simple solution, but I'm sure I can improve it without too much effort, and also make it a bit more configurable (line thickness etc.).
