I don't see your idea working very well, but you can increase you odds of success by digging the hole bigger, and filling it with aged horse manure.

If your choices are compost or peat moss, go with the compost.

It may be a couple of years before the wood breaks down. You can speed things up by burying the whole thing in aged manure or grass clippings.

More thoughts...

If you can rent a chipper to chip the branches, they will decompose faster.

If you can turn the pile, it will decompose faster.

If you fill your worm hole with compost or aged manure, just top it off with more compost or manure from time to time. You won't need anything special to feed it.

If you add some high nitrogen organic matter (grass clippings, manure) to the wood pile, after a while, it will become your worm food. I think that is what you have in mind, right?

Don't bother with the clay.

The clay is for the eventual garden soil.

The brush pile is huge. I could grind it with a bulldozer driving back and forth over it once it rotted a bit, but no other feasible way.

The suggestion of aged horse manure is a definite "go!" Thanks.

Though I would be tempted to add sand to clay soil, I don't think I would add clay to sandy soil. I could be wrong.

Haven't ever tried "hugelkultur" but I might be tempted if I was starting over.
Bury the wood and cover with your greens and browns. The worms would do well in this environment I would think.

Here is a link that might be useful: Is this an option?

"hugelkultur"

Thanks. Never heard the term but it is essentially what I am planning to do with a bunch of bushhogged rotting wood that I want eventually to add OM to sandy soil in a large ring around a big jelly plam (Butia capitata).

A brush pile from electric-line right-of-way clearing next to the palm was not chipped by the linemen, rather they ran a bushhog over it a few times. (I see this more and more.) I've piled the wood and now will bury with leaves or grass clippings.

This idea may be out there but why not throw it out in the ring. Have you heard of straw bale gardening? Why dig down when you could build up?

Leave the sticks and leafs on top of the ground and place a layer of bales over that. Add compost on top and follow the typical straw bale garden plan.

I did this to build a raised bed and most of the straw was broken down after one sseason. In a few threads some have talked about worm loving wet straw so that may help after it cooks a bit.

I am looking at this as a veg gardener who really likes raised beds.

Like sbyce says renting a wood chipper and mixing the chips with manure, grass clippings, etc., and burying them in a hole is a good way to start. You can continue adding other materials as mr_yan suggests to make a raised bed or 'lasagna' garden. I would suggest however, you might want to place a 4' x 8' piece of scrap plywood and/or a tarp over the pile to hold in heat. Periodically turning the pile with a potato fork and soaking the area with water helps the decomposition process.

You might even find native worms coming to dine. Add some of your granular soya meal, cottonseed meal, and alfalfa meal to the surface of the pile and you might find some dandy fishing worms under the board, if your into that sort of thing.

Grinding a brushpile that (1) is already several big dumptrucks loads in volume, and (2) getting bigger all the time for another year or two, is simply impractical. It is going to rot in place. I spray the top of the pile with high-N dissolved fertilizer now and then to help it decay and throw a lot of rotting-wood "innoculum" onto the fresh added wood, but the pile cannot be reworked physically no matter what the benefit. The worms I was hoping could help with the final breakdown and mixing at the detritus base and soil surface. Most of the initial decay will be in the brushpile above them. The clay sprayed or tossed onto the wood I hope will help make an eventual loam layer at the soil's near-surface.

More-perfect composting is attainable by a person at the cubic-yard scale perhaps, but without power equipment not at the tens of cubic yards scale. Especially when the real job at hand is felling, cutting up, and hauling the trees.

With such poor sandy soil I simply did not want to burn all this potentially useful organic matter.

Considering the size of your project, I think you are doing the best you can. If there are native worms in the soil, you may not need to bring any more in. If you want to bring in worms, give them plenty of manure or compost to start out in. As the leaves decompose, they will add to the worm food. The bark and smaller twigs will become food next, and so on until the larger pieces are able to break down. The whole thing may take several years. You will also find other critters, like wood lice, moving in and helping out.