Try typing this in the browser console (might need to reload the game first)

nethack.apply_tileset('http://cook.web.eschelon.com/pictures/geoduck20x12.bmp', 12, 20)

I felt that a complete ASCII port worth a separate project, yet it might not be that interesting as we've already got NAO.

But if you guys feel that a ASCII tileset is ok, I'm happy to add it. How about this smaller version?