Here is one of my setups with Java Moss growing over the background - a single crab is living in here:
You can indeed grow aquatic mosses and many plants emersed in a humid vivarium. I like to experiment with aquatics, in the big crab tank has Riccia fluritans (aquatic liverwort) growing up and out of the pools, java ferns and other "higher" aquatics growing in only 1-2 inches of water with leaves emerged and they are creeping up onto "dry" land. As long as humidity is high enough and the stems are sturdy enough the plants don't seem to care. Java moss spreads/establishes very slowly underwater but this background only took a month or so to settle in and get growing.