You get your general color notifications on the keyboard itself but not an indication of how much battery life you have left.
I noticed when playing with widgets in iOS 9 beta that there is a battery indicator you can enable and see the status of your bluetooth devices. Assuming the indicator is correct, this is exactly what I was looking for. Hopefully they keep this option when iOS 9 goes GA.
In case someone stumbles across this post, has iOS 9, and wants to enable this feature, here is how you turn it on:
Open Notification Center by swiping down from the top of the screen.