Sounds good - just let the water table vary so that the LFS can dry a little every once in a while. Otherwise you'll just end up with a big tub of brownish-green muck. Or you could put some gravel or sand under the LFS, so that it isn't constantly submerged. In my experience, substrate isn't strictly necessary for all aquatics (U. gibba and inflata are the ones I'm experienced with) but it's such an enormous help that you might as well call it a requirement. Also, the substrate doesn't seem to need to be organic - I have U. gibba growing on live sphagnum, dried sphagnum, sand and gravel. All of them flower when triggered by density and low water levels. The only difference I've noted is that the containers with dead sphagnum seem grow more densely than the others; my guess is that the extra nutrients in the water are responsible. In particular, my Nep terraria, which are wet on the bottom with a mixture of U. gibba, live and dead sphagnum, produce prolific amounts of gibba when I leave them alone for a few weeks at a time.
~Joe
