Can I run a ground wire of a single circuit to the panel outside the house?
National Electrical Code 2014
Chapter 3 Wiring Methods
Article 300 Wiring Methods and Materials
I. General
300.3 Conductors
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded
conductor and all equipment-grounding conductors and bonding
conductors shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or cord, unless
otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

The hot and neutral wires should be in the same raceway to reduce Inductive Coupling. You'll see using the Right-hand rule, that the magnetic fields created by the hot and neutral wires tend to cancel each other out. Thereby reducing the affects of inductive coupling.

I = current flow direction. B = direction of the magnetic field.

The ground wire was required to run in the same raceway, to provide a safe path for electricity to travel in the event of a short. If one of the wires sheathing becomes damaged, it's likely the exposed wire would have come in contact with the ground. The same way an equipment ground provides protection at a consumer, running the ground in the same raceway provides similar protection the entire length of the circuit.