This is broken in the Yosemite public beta (probably do to some menu bar reorganisation?) Anyway, I tinkered a bit but couldn't fix it. The best I came up with was this Applescript:
tell application "System Events"
tell process "SystemUIServer"
try
key down option
click menu bar item 1 of menu bar 2
key up option
on error
key up option
end try
end tell
end tell
Note the use of the try block. In the current version of the workflow the option key gets stuck pressed down, preventing you from typing normal characters until you restart System Events (or reboot your Mac).
I can see the script above clicking the Notification Centre icon, because the icon flashes blue when I run it. It doesn't actually toggle Do Not Disturb though.
