If the rooms you are thinking about get sun every day, you might consider thyme instead of moss. Elfin thyme and creeping thyme feel wonderful on bare feet. I have concrete stones covered with these two varieties. Thyme doesn't grow fast, but spreads very year. It's tricky to grow from seed, which is probably the only way this project would be affordable. I've always planted starts, cut them in half. Worth trying from seed. Thyme doesn't need near as much water that moss would and it handles light traffic well. I wouldn't try it in a kitchen, too much traffic. Very interesting idea, you've got me thinking about using it in rooms that transition from indoor to outdoor.