While I was pulling things apart, I decided to splice into a fused always-on +12v connection for the panel that I decided not to use for powering my cabin-fan mod. This may not be the final location of the panel.
I expect I can get at least 8 hours of good sun per day and another 4-6 hours of indirect light if I get a crap parking spot in the sun, so I'm guessing 0.5-1.5Ah per day. We don't drive it much on weekdays. Should be safe, right?
I suspect keeping the +12v topped off may have a noticeable impact on mileage, as even turning up the radio too loud can cause up to 4 bars of constant regen.