You cannot feed a Divine Favor potion to a summon or associate currently because the buffs will always default to the summoner.
It's script defaults to itself, unlike Ghostly Visage, which uses the target of the spell (like most spells do). Limiting it to self-cast is something that's already done in the 2DA, so this solves that issue and wont allow people to target anything other than items in their inventory, or themselves. To prevent possible issues and add some redundancy, it could have a "if not associate, then OBJECT_SELF" to default to otherwise.
I think the same goes for One With The Land
Can't feed undead summons/minions/henchment inflict potions either.