[brucepick]

Any other words of wisdom on finding DC-DC voltage regulators?? I'd likely have a battery pack somewhere between 13-18V and would want it to put out a steady 13.5 or so, to mimic a running alternator.

OTOH,
I'm still leaning towards a 4S (four cells in series) LiFePO4 pack that will give a nominal 13.2V. Several of those in parallel. Have them just run the headlights and maybe the heater fan if I can wire that up. Switch the small battery pack in manually when needed or with a clever relay circuit as discussed earlier.

The benefits: I'm making no headway on finding a proper DC-DC converter for this application. And, this method would avoid any losses from the DC-DC regulator that would modify the higher voltage battery pack. I'd be able to start using the battery pack with the number of cells I can afford to get, and add more in parallel later for increased AH. When it gets big enough to power the whole car for 3-2 hours, add battery disconnect switches to switch between the Li Fe Phos pack and the oem starting battery.