iOS/OS X internals guru Gwynne Raskind tipped me off to a much better way of creating associated object keys.
Until now, I've been doing this:
static const char nametag_key;
return objc_getAssociatedObject(self, (void *) &nametag_key);
Turns out there's a much easier and better way. That's because Apple's selector implementation uses a fixed address. That means you can de