Since the window system exposes all windows including their attributes(position,size, etc) via the file system, at least some of the tiling WM functionality could easily be implemented with rc scripts. Being a dwm user myself I planned to do so, but after using rio for some time I found this completely unnecessary since you can place your windows very conveniently with the mouse in the first place.
Unfortunately I'm all talk at this point. I have to set up an actual plan9 machine at some point and start working on it if I were to get anywhere.