I wish I understood how the Amiga disk API's worked.... (need to read up).
If the ROM supports SCSI-direct can you form any SCSI command you like? i.e. READ12 commands?
Yes, that's what Direct-SCSI is made for (actually the command's name is HD_SCSICMD. The name Direct-SCSI was inspired by other people. I first read it in the PFS3 manual).
However, there might be drivers which check the SCSI command before they send it to the bus and this check might refuse commands which weren't known when the driver was written.