I think you have a shot at this, and please do post back on your experimentation.
Read up in here on the two different versions of the Z400/Z600/Z800, and how to look at their boot block date in BIOS to tell if you have version 1 or version 2 (which is officially known as revision 1).
Then, read up on the processor limitations depending on whether you have the earlier or later version. If you have the original version then you are out of luck due to a chipset limitation.
Then, there is info on whether a processor is listed in your workstation's QuickSpecs. Generally if it is not listed it it will not work due to it not being present in the "microcode" part of BIOS. Just because it theoretically should work does not mean that it will. This rule has a very few exceptions, and this may be one of them.
Long and short of it is.... if you have the revision 1 motherboard (the later version) and you load the latest BIOS you might just get lucky and have it work. You'll need to learn about "Performance" heatsinks, too. If the motherboard does not see a high performance heatsink it will refuse to proceed. You can spoof the motherboard on this, however, but it is best to use a true higher performance HP heatsink in this situation rather than the wiring spoof.
If you have free access to one of those processors and a Performance heatsink I'd recommend trying it. You won't hurt the processor or the workstation. There are some hints that this will work, despite it not being supposed to in the Z400 revision 1. Everything above must line up correctly.....
Let us know.......