libwayland being a reference implementation does not mean its not implemented. It just means that the specified thing is the protocol, not the library API. This is similar to libX11, its just a reference implementation, and other implementations like escher (a pure java X11 protocol implementation) exists. Most wayland servers will still use libwayland.
There is nothing special about multiple monitors in wayland. The KMS/Mesa drivers allows setup and rendering to multiple monitors fine, then its up to the wayland server to decide what to draw on each monitor.
then its up to the wayland server to decide what to draw on each monitor
This is the kind of "mechanism without any policy" thinking that I was hoping to escape from. What does Weston actually do in the presence of multiple monitors?
There is nothing special about multiple monitors in wayland. The KMS/Mesa drivers allows setup and rendering to multiple monitors fine, then its up to the wayland server to decide what to draw on each monitor.