There does not seem to be a config setting for mapping the mouse button actions for the root window.
Additional information: middle is tasks, right is menu, but these should be easily redifinable.
this is controlled by xfdesktop, not xfwm4. i'm doing a lot of work on the desktop presently, and the mouse button actions will (possibly) be configurable later. at any rate, please use the xfce4-dev@xfce.org mailing list for feature requests and leave the bug tracker for actual bugs.