The SCSI address #0 is a special address, but it is not absolutely reserved for system use. Not taking the Startup Device cDEV into account, if the Macintosh sees a storage device attached to it that has a SCSI ID of 0, then it will wait for that device to come up to speed to see if it is bootable. If it is bootable, the system will start up from that device. There is no other special significance attached to the #0 ID, so you can set an external device to it if you like.