I worked as a ramper for SkyWest/United/Continental, and part of the time I handled bags. Having handled bags myself, and having seen how unbelievably reckless those bags were treated by *all* airlines when behind the scenes (or even right on the ramp), I will never check my Mac Pro unless it's in some sort of anti-gravity über-darkmatter box.
I've never had to put mine on a plane, and instead transported mine back and forth from Colorado to Texas for a big job in my own car. However, I do foresee the possibility of transporting it to the UK one day, and I probably won't go by sea, so I think about this from time to time.
I've decided that if I had to take it on a plane, I would get one of those padded luggage bags made just for the Mac Pro...
something like this. I'd wheel it to the gate with me as carry-on, despite what any airline employee told me as far as "that's too big." Then, shortly before the boarding process begins, I'd ask for a gate check of the bag, so that they hand-stow it in the pit, and pull it out by hand for return at the gate when you land. At least that way it doesn't move on those open carts (the ones with plastic curtains that don't stop anything from falling out, LOL) pulled by little tugs around the airport. *That* is the *last* place you want that box, because they fall out and get run over by other tugs, dropped off the cart, thrown onto metal belts and my god, you just wouldn't believe how they're treated in the baggage areas... particularly heavy-as-all-get-out boxes like your Mac Pro would be.
There's still a chance that it will fall off the belt loader as it goes in and out of the plane's pit, but you have no choice, since they all ride on that belt loader one way or another, because no lazy bagger is going to carry that heavy box up the belt by hand.
My other thought was shipping via some insured method, like FedEx or something, so that it would be covered in case of damage.
I have to say that if you can rent, swap or buy a Mac Pro on the other side of the world, it might be worth saving the hassle. You could remove your CPU tray, GPU(s) and drives, and transport those in hand luggage pretty easily, and just insert them into another Mac Pro of the same year. For the price of that padded bag I linked, you might even find a base Mac Pro for use as a shell.
Lastly, if you really strip it out and wrap it nicely, and pack all the internal parts separately, I bet the empty Mac Pro case is fairly light, and would check ok through luggage. It's the weight of a loaded Mac Pro, falling from ten+ feet onto solid concrete that will kill it, but lightened, it would be ok in the Apple box, in my belief.
I feel for you, and hope you find a good solution! Let us know, because I would love to hear about it!
