This is a spell that the engine casts internally. Although there is no SPL listed, I know that this ability uses the #0 projectile and belongs to the NONE school of magic, and is of the NONE spell type. The controlling function applies the Control Creature effect, and the blow-up the Kill effect. Unfortunately, way too many other spells, abilities and weapons do too, so latching on to those features to change how Turning works isn't going to work. Is there anything that can be done with it? And is there any problem with changing spells and weapons that use the #0 projectile so they use another one, also instantaneous and invisible? I don't think there are any mods that prevent or block or reflect the #0 projectile, for just these reasons, so there seems to be no risk in a wide conversion. Perhaps PRO files that use #0 in Secondary Projectile can be patched with the replacement, too.
This is all a long shot, but, since the patching is only going to affect existent SPL and PRO and ITM files, the invisible SPL of Turning is going to be left alone in its own little world with #0. Then it should be possible to patch undead so they react to that projectile in more interesting ways. I have a notion of a struggle of wills between turners and the more powerful undead - liches, vampires, spectres, where the result may be that the priest himself gets "turned" instead. I can already do this for evil-aligned priests with a reflection of the Control creature effect, and it is very nice - they go red and turn on the party. Then I could change a spell like Remove Curse to remove that, and so on. This part is easy. But I would like to bounce back also the more usual fear effect and the good and neutral-aligned blow-up. Testing this last with a high-level Yeslick was one of the more entertaining spectacles I've got to see lately.