Pushing buttons with a mouse = the devil.

To the best of my knowledge there is no way to push a button on one instance of WoW with a mouse click, and have it relay a command / macro to another instance of WoW.

The closest thing you could do to this, providing you've got a lot of time for testing and patience, is setup a third party application (Like hotkeynet) to pass your mouse clicks, and setup your alt's buttons in the exact same place as your "Dummy" buttons on your main. However, I did find once by accident that while using HotKeyNet if you tell it to pass your left mouseclick to your main characters screen, then click on it, it will indefinitly pass left mouse click for the rest of your life, or until you find a way to crash Hotkeynet (I had to hard reset because I was unable to get windows to do anything else). This in theory would happen with any program, and therefore make mouseclicking in that way all but impossible.