I think that a fire lane, not in use but blocked by snow, could still be a grey area if some official wanted to be in your face. I didn't come across that example when I did my pre-trip research. I always check each states policy on overnighting etc. Alaska's policy seems to be, unless it says you can't, you can.
Alaska and Texas, my favorite states.
It was even allowed at the rest stop near Denali, which was really convenient. They did charge a few bucks to overnight but, without having reservations at the peak time, it was great for us.