Create a shortcut which moves the selected image(s) in Library to the top of the album. This would be quicker than manual drag and drop in large catalogs which requires scrolling through thousands of images.
While waiting for Adobe to implement something better, here's a recipe for moving photo A to the top of the custom order that, with a little practice, can be much easier than trying to drag it up through thousands of photos:
1. Click on A, selecting it.
2. Drag the scroll bar to the very top.
3. Shift-click on the first photo in the collection, so all photos from the first to A are selected.
4. Type left-arrow. That scrolls the grid back to A.
5. Cmd/Ctrl-click on A, unselecting it.
6. Click and drag the photo immediately to the left of A to the right of A, dragging all previous photos to the right of A and putting A first.