Sorry to drag up an old thread, but I've found nothing more recent than this discussion and I've also checked the KB and 8.3 Best Practice CIFS guide without finding anything useful.
I too have been trying to overcome this problem today, and here's what I've come up with instead of using API calls and works entirely within the CLI.
# Create a QTree in the SVM root volume
qtree create -vserver VS1 -volume VS1_root -qtree SUBFOLDER
# Mount a volume to the QTree
mount -vserver VS1 -volume volData -junction /SUBFOLDER/volData
This allows you to create a second level mount against a folder (QTree) rather than creating a stub volume and also without resorting to API calls or PowerShell scripts to overcome it. Hope this is of use to someone.