I would like to have a toggle for the battery saver mode feature, but I don't think it's possible at this stage. Perhaps someone knows better?
There isn't a built-in way to achieve what you want.
But since GDR-3 apps can offer to your the possibility to pin the screen rotation and the battery saver mode.
It's just a link to the feature and you still need to toggle the button.
One app I can remember do this is Insider from DAONE (DISCLAIMER: I use the app but I'm not related to this and/or know who is responsible for)