@Michael Holtman - I have done similar things, but what do you plan to do with it after it had decomposed and re-filled a number of times?
Some ideas if you haven't thought that far:
1. Plant a seedling beside it and keep filling it to
feed the seedling.
2. Plant potatoes in the partially decomposed mulch. That's something I've actually done. The first year it was too dry and not decomposed
enough so I didn't get a crop, but the
roots would have still helped decompose the mulch, and this year it looked quite happy, but I had added a layer of veggie scraps near the top in the fall to encourage worms to visit.
3. I've done similar but digging into the dirt to try and attract worms and improve the soil a little deeper in areas of my property where there's barely 1/2 an inch of soil over mineral soil. The plan will be to plant a tree seed or seedling in the spot eventually. These are usually smaller than 3 ft diameter, but I could make the top wider than the hole now that you've given me the idea!
Good luck with your food forest.