There are such implementations, working on the Pi hardware and everywhere else. You just need EGL for wayland to work (low enough requirement).
The special implementation for the Pi is simply there to take advantage of the special features of the Pi hardware: the Pi has a hardware compositor that you can use instead of standard software one. This has nothing to do with the driver, and cannot be applied to other devices.
It will work with any implementation, but it will just work better with a custom one. The only difference with wayland is that you can actually provide such a custom implementation (=> with X11 they simply couldn't, with Mir I don't know).