The concept of "HP certified" is somewhat fluid. In some cases the hardware just won't work. In other cases HP never tried that hardware. Both would be not HP certified.
The later HP Quickspecs for the Z600 mainly deal with the Z600 version 2. In the Z600 quickspecs v26 there is this statement regarding memory and the v1 versus v2 capabilities of the workstation. Emphasis added....
"NOTE: Although all of these memory configurations incorporate 1333MHz memory modules, the speed
at which they operate is dependent upon the processor. Support for Registered DIMMs on the Z600 requires a systemboard with the C2 revision of the Intel 5520 chipset (which is the v2 version).Two methods are available to determine if a specific Z600 system has the C2 revision of the chipset. 1. Use the BIOS setup menu to access the “Boot Block Date” from the “System Information Menu”. All B3-based systems will have a “1/30/09” date and C2-based systems will have a "01/07/10" date. 2. HP Performance Advisor SW can be used to determine the PCA ID, which is reported by Performance Advisor under “System Configuration” and “Baseboard ID”. All B3-based systems will have the ID "0AE8h" and all C2-based systems will have the ID "0B54h"."
The same holds true for the Z800 v2 workstation, but for some reason the Z400 v2 simply cannot run ECC Registered (buffered) memory in any test I have done.