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.