I have the same issue at a boarding stable.
This is not perfect, but the best I came up with: I use pergolide capsules. Place that and the Equioxx tab in a very small mini 4 oz snack size air tight lid plastic containers from Glad , along with other dry supplements. I add Equine flavor fix for peppermint flavoring. Sometimes Ive used the powdered greens used in human smoothies, to add flavor.
Not 100% the horse will eat the pills, but most of the time it works.
Yes, the pill/capsule is exposed to air...but if the boarding stable worker wont do it individually for each horse, this has been the best method for me.
I'm interested to see what others have to say
Victoria , Maryland, joined March 2020