That is great news, and I do have some input on your new question. Here is the long answer:
The HP Restore partition/discs often are for more than one type of workstation (listed on the disc label), and the builds for each of those results in the HP OEM COA being identical (if you probe the COA serial number with appropriate tool). However, that same HP OEM COA will be on many other workstations around the world built with other Restore disc sets. So, it is not just the COA that counts..... there is another unseen part of the OEM cross check process that looks at the hardware and sees if it is HP-approved for that build. Reportedly it checks some unique ID feature in the workstation's BIOS. If all checks out OK the build will become "activated" even without any internet access back to Microsoft.
A build that checks out OK will run as a clone on another HP workstation that also checks out as OK, and that is what HP does. You can too. When these builds are created more drivers are present in the drive's hidden archive than are needed for that particular hardware it is running on. That is the case for all Microsoft OS installs. If you take a clone build and plug it into another similar workstation on the approved list it generally will boot up, find any needed alternative drivers, and require nothing more than a reboot to work fine, auto-activated. This assumes the build was made from a Restore installer that is for both types of HP workstations (which is exactly your case). Both will have identical HP OEM COAs, and those COAs will be identical to my HP OEM COAs here on my Restore based Z600/Z620/Z640 builds, and others.
Next step: If you take your Restore clone and put it into another HP workstation that is similar but not on the approved list for your Restore installer it will generally still boot up and find proper drivers, do its reboot, and yet will not be "activated" when you navigate to Computer/Properties. It will generally give you 3 days to enter a non-OEM COA. For W7Pro64 that can be from a Microsoft System Builder kit you buy, for example, or one may have come with the original workstation.
W10 will be different..... a workstation that has been W10-activated has its UUID (and other things) registered centrally with Microsoft. A W10 master build for a Z620 will be able to work on any other Z620 that also is W10-activated, independent of the HP OEM COA system.
Short answer is that your idea will work, no problem.
You can see that those Restore discs are valuable and important, and need to be filed away carefully so you can find them later. If there is a Restore partition on the original drive that came in the workstation I like to capture the Restore software onto a USB thumb drive (a 10GB high quality one is what I use for each workstation), and I file that also with the Restore discs. You can only capture once from each Restore partition, but a clean install from that thumb drive creates a fresh Restore partition that you can capture a Restore USB from, once, again. So, you can have >1 backups if you wish for safety. You can use a small fast stunt SSD for that type of thing.