I was able to get service diagrams of the Spark EV charger so now I need to try to get the same for the Bolt and compare. I imagine they differ only in physical size, which is not a trivial problem.
Hopefully, the actual charging smarts is in the charger itself and what we see on the dash display is fed by that. (This is how the Zivan charger I used on my last homebrew EV worked. It had pins that communicated status via voltage level changes, etc.) Or, the dash display would just be wrong (estimating e.g. 7 hours when it actually will take 3.) But I suspect the dash display is informed by the actual current going to the batteries because the estimated time of full charge changes gradually as the battery current goes up. (Try it: plug in with the 110V EVSE, note the estimated time. Change it to 12A, watch the time adjust a few times as the rate ramps up.)