At first I thought this might be some variation on the extended attributes that can be modified with the xattr command line tool. However, I've staged several tests, and the files don't seem to have any special attributes while in this mode.
Is this accessible at all from the command line, or is it only possible from within some cocoa api?
You can do this in plain shell script using the xattr
command.
xattr -w com.apple.progress.fractionCompleted 0.654 file.ext
If you inspect the xattr of an in-progress download you'll see it, here I'm using the xattred app: