Unfortunately there's not a great alternative way to do this, the old Windows XP/2003 installers were designed to only pull drivers off an A: floppy drive and nothing else. Vista/2008 onward has the ability to read off any USB mass storage device so things are easier there. If you can find a USB drive that knows how to emulate a USB floppy drive, that could be a workaround. The other option would be to keep a system image created on a different system and use sysprep to slipstream in the SCSI driver that you need, though that is likely more effort than it is worth.