I believe it would be MuCo(Aq).
I would allow the spell to work under any circumstances that there was a mundane object: head, rock, pinapple, pumpkin, bag of flour, you name it.
I asked my SG specifically "If I make a ReTe spell to turn a blade into a pretzel shaped blade does it actually twist into this shape...or for that matter if I change the blade into a long coil of wire?" The answer was no, one second it is a perfectly useful blade the next it is well rather less useful. Using the logic of this there is NO reason to add complexity due to the fact you might under some circumstances penetrate an object. In principle the spell could care not a whit.
You might add complexity for other reasons...such as the fact it is an extreme modification...that would seem to me to be an almost Rego thing not really a Muto... I could infact see you doing exactly the same sort of thing with a ReTe spell.
You might even require a finese roll or somehting like that since the helments would be anything but uniform in make (under normal conditions anyway they are more likely a fairl motley collection).
But the fact that the spikes go into someone head, or a rock or bag of flour I don't see how that effects anything.
I'd be inclined to make "Group" a bit harder then normal given the likely variation inside the group and also add a finese roll. But I'm not sure either can be justified by the existing guidelines.
If a SG thinks its too powerful then I don't see a problem with them saying "Nope, that doesn't work." But strickly by guidelines it should work as the creator intends.
I find this sort of spell pathetic since killing mundanes with magic is so trivial that my mage generally speaking refuses to use magic when fighting mundanes.