This feature was considered when we designed Primo Ramdisk. It is possible to not assign a drive letter for the ramdisk. Actually our ramdisk creation process does not depend on the drive letter. However this will introduce some minor issues and bring additional work. So currently we simply require a drive letter.
Alternatively, you might follow the steps below to get no drive letter,
1) Assign a drive letter first to the SCSI ramdisk when creating.
2) After the ramdisk is created, go to the Computer Management (Control Panel->Administrative Tools->Computer Management), and then remove the drive letter in Disk Management.