I got the Magic Trackpad plugger boards back from OSHPark today. The only minor change required is that it turns out that the entire inside of the battery cavity is not conductive. You have to use a stiff piece of wire and sort of jam it in the threads near the outside edge that would mate with Apple's metal plug in order to make the negative contact. So I will revise the board, moving the negative output terminal into the back corner to make that more convenient. But apart from that, it couldn't have gone better. The boards are the perfect size to center them inside the battery cavity, insuring that the positive connection will work. And the plan at the moment is to wedge a rubber test tube cork into the end of the trackpad to keep it all together. The cork will hold the negative wire in place and provide strain relief on the USB cable.
The boards will go into the store immediately. They come with the surface mount components assembled, as well as the solid core 22 AWG contact wires. You just add the chopped-off USB cable of your choice and the cork. It'll pay for itself easily in less than a year's worth of batteries!