I forgot to say that your area is probably low humidity and very dry, just like we normally are in AZ. If you did the moss thing, you could mix in water holding crystals into the soil that went into the walls that were mossy. Your wife could also keep a spray bottle handy and mist it when she thinks of it. Keeping it damper than normal should keep it growing. I once brought some moss home from a trip to a northern AZ forest, and I put it into a terrarium. I kept that moss going for years by keeping it damp.
