Custom sort is available in albums, but it's not included for Smart Albums, or for those special views like Recents or Library.
In an album, when I need a custom sort, I just grab a picture and drag it to the place I want. You can select several pictures, grab them, and drag them all to a spot between any other two pictures.
Sometimes it's a trick to drag pictures to a position off the screen, but you can make the thumbnails their smallest so you get the most on the screen. You may have to "put them down" at a spot on your way, scroll the screen, and pick them up again. Better: if you're dextrous with a trackpad, you can even scroll and drag at the same time.
Let us know if this helps, or if you meant something different.