On the right - a piece of junk I pulled of eBay for 40 euro as MBX-147 motherboard is perfectly working piece from Sony VAIO SZ series. I also have a full SZ laptop in a pretty bad optical condition, that is also perfectly OK.
Both models feature a tiny 30 pin debug connector that seems to provide a direct access to embedded controller and it's firmware. Maybe it is even possible to feed firmware on its own? The embedded controller is a small chip that controls the inner life of the computer. VAIOs use Renesas H8/2117 processors that are pretty powerful and independent, making the machine to boot and work fast. This functionality is provided on newer laptops by the controversial Intel #ManagementEngine.
Small update: having attached the keyboard (which also contains wireless, HDD and battery LEDs) it turns out that after plugging the battery and having applied AC power, the charging LED is glowing orange-red! So at least charging circuitry is somehow responding.