I suppose using X11 on OS X it must be possible to use another WM than the one provided by Apple (quartz-wm). After all, they're providing twm, so there must be a way to run it.
I think this is exactly correct. My understanding is that you can run a window manager like XMonad (and, in fact, I've met people who do) but then you will only be able to use X programs and not native Mac ones. If all you need is a terminal emulator, a browser and a text editor then this would work perfectly.