Im very new to this forum so forgive my ignorance or incorrect information. (experts, please reply with correct answers or hints in the right direction)
If im correct you should either get the VSC (wich sound very logical to use to find those service areas)
from the firmware or software tools made for your specific hard-drives. these can be very vendor specific.
Firmware can be obtain in a few simple ways, some even cheap or free.
Getting the commands to actually work from one of the three ways to communicate with the harddrive is another story.
First firmware, either get the official or unofficial firmware update/repair tool and look in the binary for the firmware (try "reverse engineering").
Use a programmer or cheap adapter and try reading the onboard flashroms. BUT, not always do these romchips contain the full firmware, sometimes not at all.
Jtag, could be expensive, but can be build aswell for "free"... learn the jtag commands and try speaking to the devices on the pcb, one might be able to get the firmware.. maybe access the service area it self !!
Commands, well, you got either the firmware or some commands... connect to the harddrive and start talking to it.
Maybe use jtag, or a serial com on the RX and TX lines. There might be some ATA commands wich you can combine but this part I dont know much about.
base line is, is hardware hacking new, this would be a hard road. Software reverse engineering should also not scare you if nobody is sharing the easy parts here (Service or master commands, if they are locked behind "paywalls", reverse engineer them