Maybe the keyboard piece could contain the main battery and you could detach it and plug in from the other side, such that the keyboard+main.bat. pieces keys would face the back of the part containing the display, and the keyboards underside would be the bottom of the device when used as a tablet. To facilitate this transition, there would be a smaller battery inside the display containing piece than supplies power while you are switching the keyboard+mainbattery to the other side (laptop->tablet form). I haven't really deeply thought about this though so there could be some issues I haven't anticipated. I just noted eg. Anandtech complaining about the rigidity when used legs crossed, if the keyboard contained the main battery this would distribute the weight better and provided more rigidity.
If you read the Reddit AMA that the surface team did a few days ago, there were hints of new accessories for the pro coming soon. They have redesigned the magnetic keyboard clip on the surface pro compared to the surface with additional connection points so it can do more - battery power was one of the things mentioned. I would be shocked if there's not a future accessory of a full keyboard with additional battery in it to convert it into a more laptop like device with double the battery power.
The question then becomes, if Microsoft did release this, when would it satisfy some of the biased IT press, or would they find more reasons to dismiss Microsoft and worship Apple?!