ash are probably not coming back in the US, might want to save some as lumber if they are good for milling, and not rotted yet, before burying.
just a thought, ash has very unique properties for woodwork,
I don't think there would be any issues using it for a hugel. Once it's dead, the lumber value starts to decrease very quickly as the log dries and checks. Go for it and welcome to Permies!
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"
The only issue I would see with ash would be if it had been treated at some point with insecticide for the borers. That stuff is pretty gnarly and would be present in the wood. It isn’t specific to the borers and thus could be bad for beneficial bugs.
"Even after all this time, the sun never says to the earth, "You owe me." Look what happens with a love like that. It lights the whole sky."
Companion Planting Guide by World Permaculture Association