Everyone over at Aquatic Plant Central is agreeing that Java Moss would work well if it were to cover the rest of the stump that is out of water, and the moss would flow into the water, which would be neat. It seems java moss grows emergent fairly well, as long as it is misted everyday, there should be enough humidity from the tank to make it feasible.
Also, I've seen Tom Barr (I believe it was Tom, atleast) used little rubber rings like airline tubing to make a little floating plant "island" and keep them inside, and from coving the rest of the aquarium. I think a similiar ring around (say, about 4 or 5 inches out from the wood, in a circle) containing Linma Minor would be kind of neat. Then when you look down on the tank, you only see the wood going into the water, and then when you look under the water, it would appear slightly shaded, with the wood. Just an idea.