Vpod on Boeing 747?

I just stumbled across a picture on the web of a 747 that has a 5th
engine mounted between the fuselage and inboard left engine, neatly
under the wing. The photo's caption states that it is a Qantas 747
carrying an extra engine for a sister ship. It was referred to as "a
Vpod configuration." I saw the photo on airliners.net, so I figured it
would be genuine.

Is this something Qantas has done, or it this common at all (carrying a
spare engine on the outside of the aircraft for ferrying purposes)?
-ben