How is the spare supposed to be locked? The way I have it in the picture doesn't do anything.
I'm not sure but I think that you need a plastic covered padlock or you will bang up your nice power coating work. GRIN.
If that nut is torqued, I don't see any way you could loosen it by turning the padlock. Nor do think there's enough room to spin the lock with a crescent wrench. So in my mind you've slowed any would be thief.
On the other hand, What if you used a long shackle lock and caught a spoke?