I've been thinking of trying pretty much the same idea - i try to drive as economically as i can but there are times when you have to use the brakes, and without regenerative braking that energy is lost.
So like you i was thinking of using a small lithium battery to take the electrical load off the alternator, then possibly when braking, switch to using a boost converter to load the alt and recharge the lithium battery. I.e. turning the car into a very mild hybrid.
From looking online i got figures of 2 - 4% electrical load, but it obviously varies a lot depending on if you're city or open road driving and what electrical loads you have: in my case the car has an electric power steering pump and in the winter i can be using lights, heater, window demisters, etc.
I've been trying to log my actual miles per gallon in order to have a baseline to compare with, once I've installed the battery, but haven't got any further yet. When i do I'll post the results.