Ok.  I just changed the ID for the tape drive to 5,

           ---------------------------------------------------
            DEV   PID                VID        REV    SCSI DEV
            ===== ================== ========== ====== ========
            rz0   MAVERICK 540S      QUANTUM    0901   DIR
            rz5   BACKUP             IRWIN      C002   DIR

Then I rebooted:

>> NetBSD/pmax Secondary Boot, Revision 1.0
>> (root@vlad, Fri Jul 16 12:48:23 EST 1999)
Boot: 3/rz0/netbsd
1468632+59056+254436 [89+68288+55224]=0x1d2684
Starting at 0x80030000
[...]

and got to where it starts talking about the SCSI devices.

rz0: 516MB, 2853 cyl, 4 head, 92 sec, 512 bytes/sect x 1057758 sectors

The one above (HD) seems to be nice and pretty as usual, but then I get the
following messages:

asc_intr: tgt 5 command phase TC zero with non-empty fifo 1
asc_intr: tgt 5 command phase TC zero; padding command
asc_intr: tgt 5 command phase TC zero with non-empty fifo 1
asc_intr: tgt 5 command phase TC zero; padding command
asc_intr: tgt 5 command phase TC zero with non-empty fifo 1
asc_intr: tgt 5 command phase TC zero; padding command

The "tgt 5" makes me believe we are talking about device with SCSI #5, the
tape drive.  I must say it does not look very pretty or promising...
 
Then I try to add a SCSI tape drive by hand
 
interceptor# ./MAKEDEV tz0
interceptor# ls -l tz0
ls: tz0: No such file or directory
interceptor# 
 
I could try rst, but from what I saw in MAKEDEV,
 
# Device "make" file.  Valid arguments:
#       all     all known devices, including local devices.
#       minimal A minimal set of devices for install disks, etc.
#       std     standard devices
#       local   configuration specific devices
# Tapes:
#       tz*     SCSI tapes, DEC TK50 cartridge tape
# Disks:
#       rz*     SCSI disks
#       vnd*    "file" pseudo-disks
#       ccd*    concatenated disk driver
#       raid*   RAIDframe disk driver
 
there is no definition for a rst device; the only tape entity is the tz*
family.