I should think you could plant pretty much anything that likes partial shade and is deer resistant. Some of the spireas might work if it's not too shady.

I thought the heavy ash residue (plus destruction of organisms by the heat) made growing anything on burn piles nearly impossible for years afterward. How low is the lowest spirea?

Well, my S. 'Golden Elf' is about 1 foot tall and maybe 3 wide. The new 'Double Play' series are all about 2-3 ft. tall. You might be pretty alkaline, so rake out some of the ash and mix in some top soil.

And of course if you thought the area would be unusable for years, the question is if you planned to plant there, why put the burn pile there? My burn pile has been in the same place for years, and weeds abound in it every year.

Ditto what the above folks have said. Add a little potting soil, rake out some ash, should be fine. Maybe even great.

My logic is this. For years growing up ( I am 50 now ) we always sown a "hot bed". My father would burn an area of garden to sow new seeds for garden. After lighting/burning an area he would then sow veggie seeds for planting. The burning would remove any weeds and weed seeds. To have a fresh start.

Just my opinion.