After looking at your setup I think you can definitely go unlit if you kept to Anubias, java fern and java moss.
Years ago I left a 20 long completely neglected except for monthly-ish topoffs, and no lighting fixture. It only got ambient light in a fairly well-sunlit room (but no direct light) and the Anubias' grew to an incredible size, a couple grew well out of the water. I had java fern and java moss in there as well, and they also grew fairly well. When I broke it down after sitting for a year, every square inch of substrate had tremendous root masses running through.
Go for it - I think that tank would be incredible completely open ontop. Good Luck!!