You can mine away stairs fairly easily using the individual block tool, though it takes some patience. I’ve been meaning to post a tutorial but haven’t gotten around to it. If you look at this post of mine here:
http://discourse.stonehearth.net/t/maratas-the-city-in-the-stone-wip/21166/8?u=hieronymous
You’ll see an example of a roughly hundred-foot-tall spiral staircase that was entirely mined out. It does take some patience and care and it’d be nice if it were more automatable but it’s definitely possible and not too hard once you figure out the appropriate dig pattern.