Transform ordering of photos in iPhoto to files
Easy.


Select all photos in your manually sorted album (e.g. using cmdA)
Choose File–Export... from the menu (or hit ⇧cmdE)
Select "Sequential" for File name, and enter a base name (which will be followed by numbers) 
In the next dialog, choose the target folder (e.g. directly on your USB drive). Your photos will be exported there, and sorting them by file name will result in your beloved custom sort order from iPhoto!


In step 3, you might also want to select other options such as picture size and file format – this comes in handy if you don't need the full-resolution versions to be exported.