The device usually has to have a "B" USB port on it making it a node (rather than a hub, which provides power). I have not seen a Win 8 device designed to be a node. It is always the root hub and so seems to always be equipped with the "A" port. I don't think the USB specs allow the consumption of power via an "A" port. So the short answer, I believe, is no.
Wow... that is actually pretty cool. I think the battery in the keyboard is a pretty novel idea. I've got to hand it to Lenovo... that are really innovating in this area. I got one of the Intel Ultrabooks in the App Innovation contest and without fail, everyone I showed it to asked me if it was "the one that the display flips around on"? Which is, of course, the Lenovo (I had to begrudgingly admit).
I noticed the reviewer seemed down on the idea that it charged through the USB port. I can understand why... the current limit is pretty low so I imagine the the charge/discharge ratio is in negative territory (so it takes more than an hour to get an hour of runtime). I've noticed that on my Nexus 7 tablet if I plug it into a "standard" USB port... it will take 10 to 12 hours to fully charge. It needs a high-current USB port or, better yet, the high-current charger it came with. I bet that the Win 8 tab would see the same behavior.