The 747 supposedly still has an underwing hardpoint meant for ferrying
a spare engine. Dunno if this is general purpose or only suitable for
the type(s) of engines used on the 747.

However, I think this has gotten somewhat passe' due to various
factors, including increasing availability of freighter variants of the
widebodies, the low probability of zorching an engine so badly that you
have to replace the whole thing rather than put in parts or
subassemblies, and more engine repair facilities in more places.

--Joe