Or even just some hornwort, which likes best just to float

It looks its best when it's bushy and on the surface like that. I would think using any floaters that reproduce by runners such as salvinia, water lettuce (which I thought didn't do so well in closed top aquariums, but I might be wrong) and Amazon frogbit will block out too much light and make your other plants unhappy.
Interesting setup. I seldom see an UG setup using all the holes available for uplifts. Do you feel it works out better that way? I thought two powerheads on either end would draw sufficiently from the middle areas in a 4' tank.
Also, I thought with UGs you needed a rather deep gravel bed - but yours seems quite shallow. Intentional? Oh and is there something under your gravel, it looks almost like Laterite or something!
I have 6 tanks running - out of those, one uses UG, a 29 gallon tank. Sometimes I put a HOB canister on there for polishing but usually it and lots of fast growing plants are the only filtration. I kind of like how compared to the others, it's not sparkling clean - it looks really seasoned. It's a great tank for livebearers. It even gets some direct sun, so it grows lots of algae which they pick away at.
So while overall I'm not a huge fan of UG or gravel in general, I keep one and am very happy with it. I can see it working very well for certain situations, but personally I'd rather have sand sloped high on one end low on the other, and put the filter on the low end so that it collects the detritus that accumulates there. IMO too much detritus collects in the spaces between gravel, and sand just looks more natural to me. But UG still has a place in the hobby, I think
