Hmm. I take it there's no going back to the CFLs?
I've grown duckweed in the past and liked it (most do not). But have not used it for light control. I suppose it will work if you can keep the right amount, and also keep the current from pushing it all to one side.
There's always old tricks like putting a layer or two of window screen under the bulbs.
Great to hear the "one-two" is working for you. In a few days when you have the final result, and if you have the time, I'd love it if you add a write-up of the experience to that thread.