How about including some PV cells in the pano roof to cover the battery maintenance electrical draw? That would keep the pack from ever reaching low volt damage. It would also look cool to see the PV cells through the sun roof. PV cells are getting so cheap and their efficency climbing every year. You could leave your car in long term parking for ever and that battery level would never drop. I don't know what the battery climate control draw is, but you might even have a little left over to charge the pack. What do you think?
And if you're parked long enough that you're worried about loss of battery capacity, you probably should be plugged in.
Specifically, see the owner review video where he does the math. It was something absurd like 500 years of leaving the car parked in 100% sunshine (read: not Earth) to recoup the cost of the solar cells.
Solar cells on the roof of a car simply cannot produce a useful amount of electricity. It's too small an area. Anyone who puts solar cells on the roof is just doing "faux green" (looking at you Henrik).
There's enough power to run a fan to circulate some fresh air in the cabin, but that's about it the only useful function, and you only need a relatively small panel for that.