I'm pretty surprised that Android Wear hasn't incorporated a feature like this yet, especially since it was added into Lollipop. I would imagine that you could set it so when the battery hits 15% or so, the screen turns black and white, has low brightness, and disables all syncing and apps and ONLY tells the time. This would go far to alleviate some concerns about not having a watch after a certain amount of time, and could be user-configurable.