I don't have a current Kensington mouse, but they still sell several with multiple buttons, OS X support, and some are programmable. Their web page is worthless, unfortunately.
What about MagicPrefs if you've got a Magic Mouse? http://magicprefs.com/
I'd be surprised, but not astonished, if Apple so tightly tied down the mouse driver that it couldn't be changed to make a mouse do what you wanted.
Cheers,
Scott.