We've tried to get the CSSEdit folks to support Sass, but so far they haven't been interested. I think the best way to get that is to have more users say they want it.
I would guess that such a change would be too big to be economically viable. Sass would require a rewrite of all the parsing code, and a redesign of the GUI CSS editor. Unless Sass gains a considerable market share among Mac users, I doubt it'll happen.
You're better off trying to convince them to include the necessary hooks in Espresso, if they add CSSEdit-style functionality to it.
