I haven't tried over wintering spinach here in northern NY but my kale came back amazingly this year. I actually have a garden of volunteers this year, lettuce, radish, mustard, tomatoes, kale, sunflowers and dill just to name a few. I am seriously thinking of putting in seeds this fall for many of my plants and see what next spring brings. Would make spring work much easier. :)
Interesting. Planting your garden in the fall. Probably would work well for the cool weather crops. Good luck.