I have a small clump of java moss that I just leave float freely in my water change bucket (it gets dechlorinated and left to sit during the week for minor topping off and aging for the Saturday water change). Whenever I get new shrimp or fish (as infrequent as it may be), I just move it over to the acclimatisation bucket. Gives them a place to hide, a piece of greenery to hang out around, some biofilm to munch on and, like you mentioned, some minor bio filtration. And since java moss is unkillable, all it takes is the ambient lighting in the room to keep it going.
This is a genius Idea!