I've been researching other options and decided to go with mini fissidens as the main cover and stay with mini pellia for accents and perhaps one area of notocyphus as a contrast.
The cinder gravel idea is a good one. I'm looking for something lower contrast than red lava as it tends to darken whatever covers it. I picked up a sample of paver base to test for reactivity. If it passes, it would be a good fit as it's grey and of varying small sizes, which would hopefully give the mini fissidens a textured look.