@neauoire initially I thought that maybe a PC would work, and it's by far the most modular and serviceable / upgradable way to go, AND you can underclock them, AMD is really reeeeeally efficient underclocked. But having to go through an inverter and then through a PSU is a killer.
Maybe once ITX12VO standard becomes more common, there will be a pathway for an APU based 12v system.
But by then I won't need one :)