This is my personal thought, but I'll have to play with it to verify that the issue is sabatoge when it sees the battery going down instead of up. As you're probably aware, when you plug in the J1772 L1 or L2 charger, the charging hardware in the PDM takes responsibility for doing the charging -- the shop manual goes into pretty descent detail on this process. But for CHAdeMO (and I presume CSS on other cars) the quick charger is responsible for all of that.
So I would be curious to experiment to see what conditions leads the PDM/VCM to shut down the "charging" process.
But yes, I suspect that the easiest option could be to just install a CAN bus bridge and lie to the car.
