Loop a 747?? Of course it can be done. It's only an aeroplane, all said and done, so nothing to get too excited about.
However, when doing aerobatics in a non aerobatic A/C, the problem lies, if the A/C is overstressed, if the manoeuvre is screwed up Is the pilot that carried out this unapproved manoeuvre, going to go to engineering and say "I’ve just been aerobating this nice A/C but I think it has been overstressed it. Can you spend so many $$$ on looking it over"? Like hell!! He/she is going to clam up. There lies the problem. This applies to all A/C, not just heavies.
I know as well as you all, that aerobatics done correctly, no more G than a steep turn is put on the A/C, but when a manoeuvre is messed up and the A/C is overstressed and not reported, then someother poor
has to fly the thing. Somewhere down the road, as A/C metal has a memory better than an elephant, it will give way.
I know of a light A/C (non aerobatics) that flew to pieces, and it transpired it had been aerobated on accasions years before.