I installed beta 1 and the latest X11 according to the recent posts. I'm using Enlighhtenment window manager. Copy/paste works as expected with xpbproxy, but only in the OS-X -> X11 direction. I never thought to check in the other direction until I read the recent post. Is this the intended behavior? John Koren wrote

This results in a error message " A clipboard manager is already running. pbproxy will not sync clipboard to pasteboard." being written in the system log.

If you are getting this message, pbproxy is still being launched somehow, even though it is not in your .xinitrc. There wasn't clear instruction from developers to testers on how to remove pbproxy when xpbproxy was released. You should delete pbproxy completely before installing the latest beta with xpbproxy. Also delete /usr/X11/lib/X11/ xinit/xinitrc.d/99-pbproxy.sh.

I then tried: exec /usr/X11/bin/xpbproxy & exec /usr/bin/quartz-wm --no-pasteboard

I think the problem is that you have an "exec" on the 2nd line along with a "&" at the end. Try removing the "exec" on the 2nd line, and then check your processes to see if it is running after starting X11. Check using "ps a" or "top". Dave