Indeed there is! You can actually set the 6 SCSI IDs to any combination of hard disks and CD-ROMs like so. These are only valid after "-sl7 scsi" of course.

-sl7:scsi:scsibus:1 "" (no device)
-sl7:scsi:scsibus:1 harddisk
-sl7:scsi:scsibus:2 cdrom

By default, the hard disk is SCSI ID 6 and the CD-ROM is ID 1.

I'm also gonna investigate if there's some way to speed up the timeout without removing the CD-ROM drive.