The Enterprise iSCSI Target stack doesn't really support tapes yet, however there is a patch available to export RAW devices via iSCSI. This only applies to a specific version of iscsitarget however!. Full support is on the roadmap, but as of this writing (September 2008) there's no guarantee when or if it'll happen.
You'll need iscsitarget version 0.4.14, eg from http://optusnet.dl.sourceforge.net/sourceforge/iscsitarget/iscsitarget-0.4.14.tar.gz
Apply the patch for rawio support from here: http://firstname.lastname@example.org&counter=1
Build and install manually
[4:0:0:0] tape HP Ultrium 4-SCSI U24W /dev/st0
Target iqn.2007-04.com.example:tape0 Lun 0 H=1,C=0,I=6,L=0,Type=rawio Type 1
Tape autoloaders work fine using the above version of iscsitarget and the rawio patch. You have to export both the tape drive devices and the changer device as the same target name.
[4:0:0:0] tape HP Ultrium 4-SCSI U24W /dev/st0 [4:0:0:1] mediumx HP 1x8 G2 AUTOLDR 1.70 -
And then use something like the following in your ietd.conf:
Target iqn.2007-04.com.example:changer0 Lun 0 H=4,C=0,I=0,L=0,Type=rawio Type 1 Lun 1 H=4,C=0,I=0,L=1,Type=rawio Type 1