I don't think the reverse function is available on all Magic Pies without a bit of modification, as the terminal on the controller
(which is usually grounded to select reverse) is not connected to anything and cannot therefore be accessed for switching.
I haven't tried it, but connecting the Reverse connection to ground (0V) on the controller should select reverse, assuming it's still enabled on the Pie's controller.
With the early Pies, the Yellow wire is not connected inside the hub and it may be possible to use this wire for switching reverse by connecting it to the reverse terminal on the controller (marked "d") inside the hub, and to a switch at the other end. But if reverse is not enabled by default, this still may not work.
Although the software for use with later controllers gives an option to enable and adjust the speed in reverse, this will only be of any use with the external cruise controller, where a reverse switch can be easily connected to it.
Check out this link containing suggestions for removing the cover:
http://goldenmotor.com/SMF/index.php?topic=2103.msg10472#msg10472 If permanent reverse rotation is required, it should be possible to achieve this by swapping the green and yellow phase wires over
(and the green and yellow hall sensor wires as well) where they are soldered onto the controller board.
Alan