Did a proof-of-concept test today. Worked very well. 51 mpg going to work, and 48+ coming home.
No fancy LiFePO4, just a big old second battery (575 CCA rating) on the floor in front of the passenger seat, connected via a couple fat cables snaked out the door frame and under the hood's rear edge to the main battery connections.
Driving in rain, with headlights and wipers and even the fan running on low, I found I could coast with engine off (EOC) for even the longest distances I usually can go - about a mile - and voltage generally didn't drop below 11.5V, once I got past the warmup period.
That's the result I was looking for. With headlights on, EOC is very problematic because the battery voltage drops pretty quickly. With the additional battery capacity, voltage drop was not a problem, so I could EOC as much as the road would allow without needing to spin the engine just to keep the battery happy.
I've looked into building a 10AH pack of LiFePO4 cells, but building a charger for the pack would be pretty serious business.
I'm leaning towards a big Odyssey battery, maybe their PC1500. It's an AGM, about 66AH. Mount it in the trunk and run a fat cable up to the engine bay, like audio guys use to power their amps. I probably could ditch the regular battery that's in the engine bay.
It can be charged via the alternator. Or for a next step, I can build a switch to cutout the alt, and charge the battery off wall current. The battery will use up my budget so I'll have to get creative about funding the charger.
Hose clamps hold the bare copper strands to the battery posts, plus an assist from Vise-Grips.