Has mesa gained option WAYLAND by default yet?

Not yet, but it is on the way as far as I know.

I think that's what's blocking this.

Not only this. There is also absence of evdev in GENERIC kernel so in
order to get Weston works you need to re-compile the whole kernel.
Also I have problems running this with amdgpu driver (SIGBUS signal
and core dump) while i950kms driver works well. Also there are small
problems like absence of XDG_RUNTIME_DIR with correct permission (700)
after installation of this port (the best way is to add this
requirement into post-install message), lack of mouse cursors and one
missing runtime dependecy (xkb-something ...)