I don't think anybody has mentioned the use of moss on some kind of substrate. If you create a sunken part in the floor ~1½" deep, lay the substrate (I've seen old outdoor rugs used as well as something as thin and simple as a bath towel) into the lowered area, and pour ground-up moss (in a blender with water or something similar) over the substrate, and the moss will grow. I've seen moss bath mats and similar things, so I don't see why you can't make it flush with the rest of the floor and use a substrate that would be comfortable to walk on.
You may want to add a floor drain to your sunken area for proper drainage and cleaning as well.
