Theres still alot more work for the EC I guess (battery, etc) ? I don't know how you would find the values needed to be sent to the EC for stuff like ACPI mode though and you can't trace what the vendor firmware is doing. Maybe a logic analyzer on the LPC pins of the EC would make you able to see how the vendor BIOS does stuff.

Stuff like detecting battery level could be done by reverse engineering the vendor DSDT and seeing which value in the EC RAM it is which isn't that hard.