Eggy,

In that sour cream recipe, I believe the thing that makes it work is the bateria cultures in the buttermilk. Since the buttermilk substitute recipes all are really just soured milk, I don't think they'll do the trick. Perhaps some live-culture plain yoghurt would work. Make sure it's live-culture, though. If it doesn't say so on the container, odds are it isn't.

This is, of course, only my opinion. I have not actually tested to see whether it works or not.