Yeah, this is still needed even with coreboot. The attiny isn't doing anything too complicated, it's just there to tell the laptop the status of the battery. In the testing I did, if you supply 12v without any of the SMBus stuff the laptop will shut off after a few minutes of use.

I'm actually looking in to making a slice battery, it should be possible but sourcing the docking connector seems to be much harder than getting the battery connector.

Would it be possible to write directly to /dev/i2c in Linux and spoof the messages? Not sure what the use case of this would be, but might be useful for something.

Yep definitely, I had this exact thought last night and found an example kernel module to spoof a battery, and one to read i2c messages. There's probably all kinds of cool things you can do with a kernel module to communicate between the laptop's i2c port and a microcontroller. The docking connector conveniently has an i2c port on it, maybe I'll look in to this in the future

It is needed as an alternative to crap noname bottom dollar Chinese replacement batteries manufactured with no quality control.