There are a number of answers about running all of Minecraft and MineOS from a USB drive. Here I'm asking about only loading/saving worlds from the USB stick. Running all of Minecraft from USB is too slow. I can copy the worlds from USB to the computer, play, then copy the files back to USB, but this is error prone.
So: can I insert a USB stick into an OSX Computer running Minecraft, and play worlds directly from that device?
A tutorial that does not answer the question is: http://minecraft.gamepedia.com/Tutorials/Playing_and_saving_Minecraft_on_a_thumb_drive#Mac Other tutorials describe using DropBox and symbolic links, but that is not applicable for use on a guest computer you don't own.