Thinking about this a bit further, if you have a massive cottonwood tree trunk that would form the bulk of your hugel, would there be a way that you might drill holes in it, or at least take a chain saw and slash it in multiple spots? Then, inoculate the holes/slashes with a fungal slurry from another piece of decomposing cottonwood. Yes, fungi would eventually find its way to that buried log, but buy directly applying the fungi and helping it to penetrate the
wood, you will substantially speed the decomposition process by years.
Cottonwood is a very soft wood. My hunch is that by drilling holes in it, you would find it completely riddled with fungi within 2 years.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf