Unfortunately, I've found that copying a sparse file from one NTFS partition to another NTFS partition creates a non-sparse file in the destination.
So, I've written a program that will take a bunch of ISOs (or any files really), and copy them to a specified destination folder. But, instead of just doing a normal copy, it will re-write the destination file as a sparse file. This can be confirmed by right-clicking on the file and choosing 'Properties'. The .ISO file will show that it is 4,699,979,776 bytes. The size on disk will be less than this, possibly by a significant amount.