Yes, X11, you can certainly use 'so' as a substitute for a clause. I started to write my original reply in those terms, but then deleted it because the example here seems to me to be a simpler, non-clausal case. Perhaps I shouldn't have deleted that part of my reply, so thank you for bringing it up.
Calif Jim - no matter whether it is a whole sentence or just one word, the grammatical name for this phenomenon is still clausal substitution. But I see your point. I hope that I am not on a completely wrong track here.