I would say that any wood material is good for
hugelkultur, the older the material the better as the the material may break down faster when buried with age. I brushed up on reading about allelopathy, and found that most of the toxins are found packed in the leaves, while the toxins can also be stored in all organs of the plant ie wood,
roots, buds, nut hulls, and many other or all other parts of the plant. It depends on which species you are dealing with. Most of the time, allelopathy (chemical warfare by plants to beat competition) is produced in the leaves, and the chemical abundance may vary from plant to plant. The black walnut makes Juglone while the tree-of-heaven makes Ailanthone and the Ailanthus has fairly recently been added to the list of allelopathic
trees. Sometimes these chemicals are beneficial for certain plants you want to grow, some plants are tolerant to the allelopathic chemical and while others will be suppressed, these ones will thrive with less competition. So these trees can be useful for food forest guilds. The location where your allelopathic tree was planted may have built up the toxins in the soil, that is likely where the soil is most impacted. After cutting your tree and moving the material, the leaves would likely have the highest toxicity moving to another location. From what I understand about plants - once the allelopathic toxin producing plant dies, it wouldn't be able to produce the chemical any longer for warfare among its foes. Just the stuff it made while alive will still be present, and the build up of the toxin as the matter breaks up in the soil.
I believe you are talking about a downed elderberry tree, and the general Sambucus genus do produce a chemical that is toxic called glycoside, a glucose or sugar based toxin. "Although the cooked berries (pulp and skin) of most species of Sambucus are edible,[7][8] the uncooked berries and other parts of plants from this genus are poisonous.[9] Leaves, twigs, branches, seeds, roots, flowers, and berries of Sambucus plants produce cyanogenic glycosides, which have toxic properties.[9]" (Wikipedia page, Sambucus). Based on this sentence, I would say that most of the toxins are not stored in the main trunk of the elder. I wouldn't worry too much about the chemical glycoside in your hugel bed from the elder based on the links I've read through.
The tree-of-heaven produces most of its allelopathic chemical in its
root bark, or the bark of the roots. There can be some toxins found in the leaflets as well. I would say that the wood of the trunk of the tree
should be okay, the spreading of the Ailanthus through its roots is how the tree spreads itself aside from germinating from seed. This makes the tree a difficult to contain, and a rapid spreader, one more reason why it has invasive qualities.
I hope my answer is helpful, and that someone with other knowledge on allelopathy can confirm or contest what I've brought here. Best of luck with your hugels!
My source material for my answer above:
Permaculture Research Institute - Plant Allelopathy
Cornell University - Allelopathy
https://en.wikipedia.org/wiki/Sambucus
https://en.wikipedia.org/wiki/Glycoside#Cyanogenic_glycoside
The allelopathic potential of alfalfa root medicagenic acid glycosides and their fate in soil environments
Evidence for allelopathy by tree-of-heaven (Ailanthus altissima).
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