Moss is a great idea. If you can find a smaller crypt, I have Crypt Petchii in 2 of my 32 oz tanks, it would add a leafy plant (which is a challenge in such a small tank). I also have stem plant trimmings in my nanos (Rotala Indica, Rotala Macaranda, Reineckii, Sunset Hygro). I've never tried dwarf hairgrass, at least not yet.
What kind of fauna do you plan to add? For something so small, most people usually go with shrimp, but I really like my Bettas in my nanos.
Do you know the color temp of your light? 20W should be plenty to grow anything if the K is decent.
The set up looks great! I've got to get me a piece of driftwood like that.