^Couldnt the items be Fling- based, like, you give them to your Poké and they use a Fling move to cause the effect on the other Pokemon, but if your Poké held it for too long they'd get the effect? Or maybe there could be a gimmick Pokemon or ability that transfers the effect of the item to the opponent. The item could be, like, a one-use sort of thing.
But spray stuff that the Trainer uses on the opponent's team? NO. Just...no.