Is it possible?
Yes, provided the cork as not been treated with anything harmful to the fish. And you will have to anchor it as was just mentioned. Easiest way to do that is to silicone it to your glass.
What would it look like?
Depends on what kind of cork you get. If you just get some thin sheets, then it will look kind of like a bulletin board. Which wouldn't look too bad I guess. If you went that route you could put up some tiny cubicles on a bare bottom substrate and put neck-ties on your fish. A small watercooler that bubbles air would be a nice touch as well as a coffee machine. You could even have one of the lights over one of the cubicles flicker all the time.
Okay, I'm getting way too carried away, back on point.
They do sell rough-cut cork bark (I'm not sure where) that people use as backgrounds. It looks a lot like the tank backs up to a tree trunk. If you want to see some examples search through some of the terrarium and paludarium sites.
Wouldn't it be hard to clean?
It can't be any harder than cleaning driftwood.
Hope that helps.
