"but I remember reading somewhere (take that with a pinch of salt) but the 747 was prep'd with a mounting point for a 5th pylon, and a 5th engine - which was mounted with an aerodynamic cover for the front."
That's pretty much it; there's a mounting point inboard of number 2 engine, and as I remember it a cover was used for the core and the fan blades were removed and carried in the hold.
As an aside in the case I was involved in, mentioned above, we took over the "fifth podder" when it arrived at our exotic destination to do a "shuttle" sector - once the pod had been removed. Despite multiple PAs explaining the "pod" we nearly had a riot when those passengers who stayed onboard during the transit spotted our ground team removing one of "their" engines......
