When I was working for Gulf Air in the early 80,s we regularly ferried engines from BAH to LHR. But it was very inefficient.
First you had to put 7 tonnes of extra fuel in the opposite wing tank to balance the aircraft, Then you were limited to (I think) 250kts which meant you had to stop for fuel in Larnaca. We tried not to do it on pax flights, or limited the load to the minimum.
I once took a picture of a GF Tristar with a third pod parked on the ramp next to a BA Tristar with a third pod, but sorry I can't find it at the moment.
The Tristar third pod kit was much more difficult to fit than the B747-200. It could take 3 hours to get it right. The B747 went off or on in an hour, and the B747 was far easier to fit and remove, and didn't have the flight limitations.