Alas, I have had similar experiences. I tried some woolly thyme as well but it too couldn't handle the heat and dry conditions even with regular watering. So I've been watching the ground looking at the native groundcovers and will likely be trying some of them between stepping stones. But none are really moss-like because it would require a much wetter environment (perhaps near seeps, shaded watercourses and waterfalls). Not sure I want moss so bad I would live in a place that gets as wet and lush as the Seattle area. I'm afraid it would dampen my spirits. :* )