If soaking is not something already offered at the barn, forget it. It’s not going to happen. There is no way to make that ask easy enough for them to do it and deal with the clean up.
At my current barn, there is no soaking. A few horses get a supplemental meal of beet pulp or similar, some with supplements some not. That the owners or the trainer does individually, every day, bringing and cleaning their own containers. Nothing of that nature is stored in the feed room for boarders.