yes half of my tank is filled with hornwort and java moss. I don't use any substrate.Well....if your tank is completely stuffed with e.g. moss and understocked with organisms, and you have some decent flow in your tank, I think you would have a powerful filtration system with just the plants.
In open filters, like HOBs, you can even replace media with just moss and some pothos sticking out.
As a bonus, you don't have to worry about algae.
Not bad for just a surface .