So I think I'm going to be able to include the battery in this project after all.
Here's the battery sitting under the keyboard (trackpad removed for illustration).
You can see the location of the power LED here too. The IR port is just to the left of that.
Anyway, the battery should fit with a little modification. I'll have to cut a rebate in the back and there's a couple of tabs on the right hand side that feel like they have nothing in them so they can be chopped off as well (I think...don't want to be cutting into lithium with a dremel). I'll also have to extend the power cables as they won't reach the way they are (it's not plugged in in the pic above).
The main issue will be temperature.
Without the battery in there the CPU idles around 45 degrees but with it just sitting on top with a good 10mm of clearance it's hovering between 52 and 55 degrees. That will increase when I lower the whole thing down so it means I have no option but to grab an unused GPU fan from somewhere and hack it in there to keep things cool. I imagine the temps will rise a bit if the CPU/GPU starts working hard (what the thermal tolerance of a lithium ion battery anyway...must look that up)
Yes I could have just left it all in the case the Air came in but there's no fun in that now is there? As for why one would want a big arse battery inside a keyboard...why not?...I wouldn't have to if Apple had a CMOS battery on the motherboard but they don't do that anymore. Plus the motherboard is designed to expect a battery and it works a whole lot better if there is one in there.
On the other hand, how many keyboard computers have a built in 6hr UPS huh?