Not without root, I am sure.
If it is any consolation, it seems that it will be handled slightly differently in Android N - the normal battery icon on the right will have an overlay "+" sign instead of the separate icon on the left. On the other hand, the top and bottom navigation bars turn red...
Frankly, I don't think Google's intent was that battery saver mode would be used routinely, but only when you were running low with no opportunity to recharge.