Brian,
I feel your pain, but these are enterprise workstations and not consumer grade boxes. You have the most recent boot block date for the Z620, so all HP-approved processors for that should work fine. A number of us on this forum have been posting on upgrading the processors.... that is different from adding a second processor (which HP does support). And, HP has not shut us out. In fact, my sense is that there are HP engineers who have done the same things we do and help within the constraints that exist in a very large corporation. The HP hardware has served us in our business very well, and many of our HP workstations have been processor-upgraded. Heres a few tips:
1. Troll the Z620 QuickSpecs for the processors, and make a worksheet..... it is not that hard. HP has a QuickSpec library, and in my experience any processor they listed as approved for the Version 1 will work in your Version 2 Z620. Not that you want to go back in time..... Your most valuable QuickSpec versions will be the latest ones, and those are easy to find. Make a list from that and figure out what is in your budget, for two (I'm assuming).
2. Do a google search on the processor(s) you are interested in and go to the Intel "ark" site (in the address). Go to the left column and click on the link with sSpec mentioned.... that will show you the sSpec codes for each of the processors. Search eBay for those, and see the prices, versus what Intel will sell you a new one for. If there is only one sSpec code then you are golden.... you will buy exactly what HP buys, but much cheaper used, especially if one step down from the very fastest in any category. I'm wary of China/Hong Kong processors. Experienced US sellers have been fine.
3. Get the right tools and the good thermal paste (Noctua, for me), and learn how to handle these processors very carefully during their install. That has been posted about in here, in detail. You obviously are skilled, and this will be pretty much a plug and play (after you pay) project.
4. Pay attention to the info in the QuickSpecs... with the Z620 not every processor can be doubled up. Some can only run by themselves, because they dont have the extra QPI link to talk with the motherboard plus the other processor.
You might get a processor never listed by HP to work. It can happen, but don't count on that. Your best list is already provided by HP in the later QuickSpecs (the latest I have here is 4/15, v46). The newer your workstation is the more expensive this type of project is, but that is no surprise.