I have just been looking into this a bit more myself as it got me thinking that I could build a mini ups using 1.2V NiMh rechargable AA batteries.
In my system I have an 18v live rail, but the UPS part would be 6V like your first post indicated, which is kind of my goal, its much easier to do conversion from 12v as you could use an L7805 regulator (I use them in the LED lights in my camper with small heatsinks) but anyway, starting with the 6v I came accross this:
The LT1085 is a 3A low drop out regulator, there is also the 1084/1083 which are 5a/7.5a versions. However looking on ebay the LT1085 isnt a cheap unit!
So looking at your original part I found this circuit for step down from 12v to multivoltages:
When I looked at it, you can remove upto and including the 3300uf capacitor on the left as thats all the mains stepdown and smoothing. Then if you look at the 'Selector switch' at the bottom it gives you the various voltages, which looking at it if you use a resistor of the value r4+r5+r6 would give you a fixed 5v output.
Theres also this simple diagram here for an LM317:
I hope that might at least be a starting point?
Have you got a way to charge your main battery? LM317 can also be used for that as thats what emergency lighting systems in offices use