This script tells the OSX UI server to click the "Add to Wunderlist" menu item. This is a horrible hack because the system preference to add shortcuts does not appear to work on the Share menu for Safari, at least with Yosemite. I combined this with a very simple Alfred workflow that is bound to a hotkey in order to trigger the script when I hit CMD-Y in Safari. It feels super crude, but it does ultimately do what I want.
Is there a better way?