What about wearing your carry-on? Has anyone had international experience, especially LHR, wearing a multi-pocket garment such as the Scottevest? Do they make you take it off and weigh it? What about carrying heavier items, such as camera, lenses, guide books, etc. stuffed in your overcoat pockets during security check, then whipping out your Kiva Keychain Tote, or putting them back in your One Bag once you're through? I know you have to remove the garment for security screening, but that's not where they weigh carry-on I presume? Do you think the difference would be significant enough to make One Bag for the OP possible?

Should the weight restrictions influence the choice of bag? I have a Tristar and worried it wouldn't be large enough. Maybe it's not for everything I'd like to have with me, but it's plenty big when it comes to toting it. I don't know the OP's gender or strength, but you can easily pack way over 8 kilos in a tristar. I took an extra bag to Rio, but could have easily spent my week down there with what I packed in the Tristar--it's just the stuff I bought wouldn't have fit coming home.