I would welcome a couple additional properties in the Font Styles editor: stretch and letter spacing. Being able to tweak the lyrics font settings this way would really scratch my itch.
I’m sure greater functionality in the Font Styles is in the pipeline, but why do you need to stretch lyric text? Usually, lyric text wants to be compact and condensed, to avoid causing distortion to the note spacing. Perhaps there’s another way of achieve the best result.
To say nothing of Frederic Goudy’s maxim that “Anyone who would letterspace lower case would steal sheep". 
Do you by any chance use a condensed font for your lyrics? If not, I would certainly recommend doing so, as this will help to minimise note spacing distortion (as Ben already mentioned). I use one of the Minion Pro condensed sets.
Ah, yes: if you want to ‘negatively stretch’ the font, you’d be best served with a Condensed style of the typeface, or a font choice that has more suitable metrics. Tracking in the whole text won’t look as good.
Yes, I was desiring the option to use negative stretching and spacing, because not all beautiful fonts (looking at Crimson Pro) have condensed styles. Of course, you could argue that “hacking” such a font in this way diminishes its beauty…
I would agree that such hacking would certainly reduce the beauty of the font. In addition, reducing the space between the letters will not only reduce readability but it will probably yield less horizontal space gain than will using a font in which the letters themselves have been condensed.
Like all customization, this sort of thing should (and can) be used responsibly. When you need it, you need it.