Once you have FaceTime installed on your Mac, you can initiate a call by entering:
- facetime://appleid
- facetime://email@address
- facetime://phone# as a URL in Safari's address bar.
Now you can double click this icon to launch the FaceTime application, with email@address already entered, and a big green button, 'FaceTime', that will place the call.
I also entered such URLs in the Address Book with a custom label, 'FaceTime' and used that to start a call.
[crarko adds: I tested this, and it works as described.]