I was referencing 250-64B, and if the size and the circumstances allow, I don't see why you can't run it on the surface. As far as the bonding issue, take a look at 250-92 a(3). Now I am sure one of the smarter people on here can give you a presise answer, but as I understand it, the AC on a fault will try to flow in the conduit because of skin effect, and not in the GEC.
