Hi Marc, no there is not currently a way to assign a custom script to the mouse buttons.

But in this case, you're talking about hitting the Alt key anyway, so I'm not sure there is a whole lot of difference between that and just using a different shortcut key to do it?

I know that several people have made the Space bar set up as a shortcut key for that particular thing, since some other apps like XSI do it in that way.

- Michael