I don’t know that that is possible. I’d recommend just start from a floppy, fix the broken system, or just I’m use the floppy’s Startup Disk control panel to select the correct partition. There were key commends to select a particular SCSI drive, but partition? Not sure
I just double-checked my MacSecrets books. Command-Option-Shift-Delete should make it ignore the HD entirely, and add a number to that to start from a specific SCSI ID. If you have an external SCSI drive you’d want to use, add it’s SCSI ID to that command to start from it.