I agree with you that Trusts should be available for story mode trials. With that being said, (Systemic and nonstory) spoilers below:

Trust compatibility for the level 89 trial has been added. No wait times, and you have a limited amount of deaths in order to prevent full resets like how it happens in Dungeons. You start off with 2 free deaths, then I think it goes up to 3 if you happen to wipe. I really appreciate how in depth they went with this and I would LOVE to see them expand on this system further in future fights. It sucks that they're not available for the earlier trial though.