Hearthlings like digging down, but what about you can dig up and make dirt monticules? Useful if you want to build fortifications on artificial high ground or make building on the side of a mountain that is way too inclined to do anything
I think if they improve the mining designations to easier designate what you want to mine in either direction they should use scaffolding similar to buildings to mine upwards. Or create a similar tool as the builder for mining which will create scaffolds, allow you to auto place items as well as save mountain hall constructions as blueprints.