$nl $- $nl will work and is slightly less messy than your otherwise similar solution.
Given that there's a simple workaround, don't hold your breath for this behavior to be changed. I'm not sure I'd even call it a bug, given that the purpose of $nl is to let the user decide where to break a line that's too long instead of Snap! breaking it at some random place. We weren't thinking about users exercising fine control over vertical spacing.
Anyway, multiple newlines are a pretty coarse control. What you should really do is take a trick from us TeX hackers and use a strut: a background-color rectangle of width 0 (or 1 if that fails) and precisely the height for the spacing you want, by controlling the font size of the strut.