IBERT does support GTZ IBERT designs in 2012.4. Attached to this answer record is a file called runme.tcl. This Tcl script can be used to generate a working IBERTGTZ design in 2012.4. Below are some instructions on how to run the Tcl script, and how to modify it.
1. Copy the attached runme.tcl script to a directory.
2. Start Vivado 2012.4 in the same directory where the runme.tcl was copied to.
3. In the Tcl console, type
If the user needs a custom IBERT design, replace step "2" in the attached runme.tcl with customized settings for IBERT. Once this is done, the rest of the steps in the Tcl file should still be executed (3,4, and 5).