Maybe not as an accessory, but possibly as a core feature of a future version of the watch. Remember that Apple just released the new 12" Macbook with a very intricate battery design. In the absence of a revolution in battery tech, I could totally see Apple thinking out of the box to significantly increase battery life.
The thought did occur to me, but if the batteries were in the band, it would seem to make it impossible to have swappable bands, and bands of the type that Apple seems to be going for.
That's a great idea. How about some induction charging on the bands? Watch ran out of battery? No problem, change out for another band. They would have a induction charging plate for you to just lay your watch or bands on.
Want to wear your watch while you sleep? No problem. If you wake up with low battery, change bands.
It's like having a camera. Just swap out batteries if you need. If only Apple thought of that for the iPhone.