Any updates? You should definitely be able to grow the fissidens and riccia on that spillway, in fact they should grow much better than in the tank I'd imagine! If they are partially sticking out of the water, then they will be able to take advantage of the much higher concentrations of CO2 in the air, which mosses really benefit from, (I know, riccia is a liverwort). Personally, I think this is a great idea, and was actually thinking of trying something like this myself, which is how I ended up on this page! I think they will really take off and look great! And with the constant flow of water over them, they should have access to plenty of nutrients. How are you going to seperate the fissidens and riccia though? I think it would be cool if you made a little "delta" where the spillway goes into, and it washes over before going into the tank, that would be more or less level with the water's surface. Then you could expand this idea! There are some mosses and liverworts circulating that really do not do well submerged long-term, but would really benefit from a set-up like this I'd imagine.