[Tristan Lall]

We looked into using a spider coupling (2- and 3-jaw versions) as a low-hassle dog shifter. But we decided it was a bad idea after a little impromptu stress testing. We gave it a good smack with a hammer, and the jaw we were striking broke off. Then we hit it a couple more times, and it developed cracks in the hub. While it might work if you picked a large size (ours was small--less than Ø2"), and were gentle with it when shifting, I don't think it's worth the risk of fracture associated with them.
The sintered iron that ours was made of was brittle, but probably absorbed vibration rather well (but that doesn't help much in a shifter). Instead, we went with something vaguely similar, out of 4140 steel--that should hold up for us.
As for castle nuts, are they designed with enough clearance between the jaws to allow another to slip in? It looks like the slots are rectangular in the little diagram. Also, what do propose to do about the threads? I'd be wary of them unscrewing themselves!