My opinion (I'm in the same game with Telas, btw), is that all combat in D&D is abstract. AoOs are provoked by creatures that divert their attention from a threat. If the creature is threatening as well, it can keep its enemies at bay enough to not provoke the AoO for minor diversions of attention.
The assumption is that combatants are constantly trying to hit each other, not swinging their weapon, then waiting six seconds to swing again. Therefore, if the cleric reaches into the occupied space without any way to threaten the warrior, and taking his attention away from the big axe being swung at him long enough to touch his unconscious ally, he might get smacked (or he might dodge, or take it off his armor, etc).
If he had a spell that could hurt the warrior, he could make threatening gestures towards him and force him to be wary enough that his swings don't have a chance to connect more often than normal.
The spellcaster knows what spell he cast and that it won't do any good to touch the defending warrior with it, therefore he will need to act differently in combat. If he wants to try to pretend that he's threatening the warrior, then let him make a Bluff vs Sense Motive+BAB, a feint, because that's what he'd be doing.
Based on Improved Unarmed Strike, it's gotta be in how the attacker carries himself. Without the feat, the defender gets an AoO, with it he doesn't. Does he have to know, "this guy doesn't have the feat, I get to swing again!"
Well, that's enough rambling for now, but that is my reasoning for arguing that an AoO.