I've used palm tree trunks to build a hugelkulture. They work fantastic. The wood of a palm tree is so different from most other trees. It's fiber-y and strong. You can cut through it easily with a sawzall (which is what I used to cut down 6 huge queen palms, rather than my chainsaw). As it begins to decay, it's tremendously spongy and absorbent --- which is exactly what you want in your hugel.
I deliberately inoculated my palm logs with
mushrooms by drilling holes and smushing a mix of wild mushrooms and
compost into them. If I were to do it today, I'd still drill the holes, but I'd make a slurry of wild mushrooms, a bit of oatmeal (to
feed the
fungi) and fresh rainwater ---- blend it up in the blender, and then just pour the slurry into the holes. Much easier that way.
The only drawback is that your hugel will deteriorate much much quicker than one built with hardwoods. I didn't mind that. The soil that has come from those old palm trunks is rich, black and beautiful.
Even better (from my perspective) is the web of roots left in the ground once you cut palm trees down. As you probably have realized, palm trees have a massive web of smaller roots, as they normally grow in poor sandy soil. The root mass helps them cling to the sand when big storms blow. My soil is heavy clay, but the trees rooted out as they normally would in sand, leaving me with a thick mass of roots in the top foot of the soil profile. Once I cut those trees down, I noticed mushrooms coming up throughout that area. I've planted apricots and a cherimoya in that space, and those new trees are thriving in the fungal network established among those old palm roots.