I asked them about this a
few days ago. The info isn't available at the moment but hopefully they can add it in the future.
Cheers!
Edited to add:
I just noticed this image on the /blackberry/features page for the notifications app:

In the left-hand image it appears that the battery-level is being displayed on the watch so the functionality must be there internally - we just need to get it exposed via the public SDK.