This past summer whenever I took a plane or train I crammed my handlebar bag into a pannier and attached two of the panniers together (I have a pair of MEC World Tour panniers with straps for this purpose) then I stuck one of my Ortliebs in as my second piece of luggage. and carried the other Ortlieb on as my carry-on bag which had all my most important stuff in it. Easy peasy.
A few times they tried to charge me for three pieces of checked baggage but they never actually did it if I protested politely. Also, trying to keep tabs on all your gear while tracking down throwaway duffel bags and stuff of that ilk isn't always as easy as it sounds when you have to pack everything onto one of those stupid carts with the brakes on them and everything is always about to fall off.
If I had to do it all over again I'd just keep the thick plastic bag I bought at the Airport in Vancouver for $5 and leave my tarp at home. That way if you're in need of a bag for your panniers and the bike has to be boxed you're set. And you'll never have to shell out huge coin for some crappy excuse for a bike bag that doesn't even survive initial packing let alone the voyage.