i fear that xfwm (xfce window manager) is not tweakable in this way, at least not without a tool like wmctrl.
wmctrl has an option
Code:
-e <MVARG>
Resize and move a window that has been specified with a -r action according to the <MVARG> argument.
it should be possible to do sth with that.
usually positioning geometry starts with 0,0 in the top left corner - sometimes it's possible to use negative values to start counting from the opposite side, so -0,-0 might or might not refer to the bottom right corner.
you might want to consider moving your launchers to the right side of the screen.
or use a different window manager - it's not too difficult to change them in xfce.