The battery state of charge is a simple PWM signal sent from the BCM to the ECU. I am unsure what will happen if the ECU is receiving a fake signal while this computer is still otherwise connected to the rest of the system. I can see at least two possible outcomes, though there may be more:
1) By sending a fake 0% battery state to ECU in place of the BCM's actual, you'll get constant forced regen even in neutral and at idle, much like when you first start the car after having the 12v battery disconnected.
2) When the ECU gets a 0% signal, it simply won't request assist, because maybe the forced charge program isn't handled by the ECU.
You wouldn't need to record a state to be restored, because the BCM will be trying to send the real state to the ECU the whole time, you'll just be cutting the wire and temporarily substituting another. Remember, I have no BCM computer anymore.
One thing I can tell you, the ECU absolutely does not control the battery level which shows up in the cluster, so at least some of the control is decentralized. My gauge always shows empty. So, it would take experimentation to see just what happens when you send a fake signal of one variable to just one place while the system is otherwise functioning normally.
In my car, when sending a fake BCM signal to the ECU, a signal of 0% results in a different engine map (speculatively a higher torque and possibly less efficient map though I don't know this for sure - but I lose lean burn). I also get a high idle of around 1200rpm because the ECU believes the IMA system would otherwise be doing a constant forced regen. Auto-stop is disabled. The car might or might not start using only the 12v starter. I'm unsure how many seconds it would take for the behavior to change if you quit interrupting the real signal but it isn't instant, as is evidenced by my sometimes getting unwanted auto-stop when when I have the Arduino change its output at 30mph - the system often still hasn't reacted by the time I've braked to the ~22mph or so where auto-stop can happen.
I'm thinking it's quite a rabbit hole. Frankly if you figure this out, you'll probably have nearly built a custom MIMA yourself.