What are the consequences of attaching an unused extra engine under Boeing 747 wing?
Operationally you would be more restricted in terms of trimming. Back when we had manual loadsheets there would be a demarcated area '5th pod operations limit'. Only the gods of the loadsheet deptartment would be allowed to do the loadsheet for these flights. I think Qantas would put a loadsheet man on board for these flights but I could be mistaken.

The flight would be re-timed as the usual 90 minute turnaround time would not be sufficient. I believe 2 hours on top of the regular turn was the norm.

An additional fuel stop was usually required as the fuel-burn was atrocious with that big thing hanging off. The weight penalty was compensated by reducing cargo.

I was working in Singapore in the 80s and we usually had Qantas doing this a few times a year. The engines were ex-SIN .. I don't know if they were overhauled in SIN or if they came into SIN on another freighter. At the time there were not many B747 freighters operating into Australia. I believe Flying Tigers had one but that was from US to Australia to SIN.. It did not do SIN-Australia.