Surface Team teases a Desktop Dock/Keyboard with Batter in the Reddit AMA
When asked: Does Microsoft have any plans for an external battery or for a thicker keyboard cover that has an extra battery? Or, if I want an external battery brick, is there a third party one I can buy that can be used with the surface.
Their reply: That would require extending the design of the accessory spine to include some way to transfer higher current between the peripheral and the main battery. Which we did...
Why put a port on the bottom for high current transfer? The only reason could be for an as-of-yet unannounced keyboard/battery combo or a Desktop Dock.