Yeah, that's what I was thinking - although I didn't think of overcharging during traction battery charging. Here is what might work, in a half-assed way: find a way to switch that sensor off and on from the cabin, and you'll have a 'manual charge mode' that can be used to top up the 12 volt battery as you near home. The best version would be idiot-resistant, by employing a 15 minute or so automatic off timer. I'm also sure that anyone who can design and build circuits could build a regulator/charge circuit that would switch the sensor off and on to maintain a voltage of 12.8 volts or so, thus augmenting Nissan's poorly implemented charging algorithm.
In the meantime, I've been using a near-daily short drive I was taking for months to see if there is a way to keep that voltage up, based on what we know. I've made sure to run the wipers once or twice per 10-15 minute drive, for just a cycle or two, and during the brief stop I made I left the car On. I also leave the headlights on Auto, run the climate control with heat on in Eco mode (usually) and the blower on 3 or 4. The result? Yesterday, after quite a few weeks of this routine, the rest voltage of the accessory battery was 12.6 volts - acceptable.