I have an oem license from Microsoft. Not the oem end-user but an OEM hardware manufacturers license. This is what I did.
Right click the desktop and go through your settings and set your computer as a developer.
You will need an IDE. Visual Studio Community is free.
Download and install the Software development Kit (SDK) and the Windows Assessment and Deployment Kit (ADK).
You are a Computer manufacturer and your company name is [whatever] mine was Used-Parts Custom Computers and Tailor Made Winstallations.
The process of making (or learning about making or deploying) a branded Custom Windows for your own Company that makes a hardware device requires you to be an OEM. This is the workaround as recommended by Microsoft in their ADK tutorial Creating a Custom Windows for Deployment tutorial at msdn (i think). Whether it's limited in duration or scope, or or not, doesn't matter. You now have an OEM license that passes the Microsoft screening.
As to whether you can access the table to alter or edit the Product key? I just found out about the product key in the eufi, we'll have to wait and see. I will follow up. The key is injected at the mfg not in Redmond so I'm thinking the mfg gets a tool. I will ask for one. But the oem should be based on GUID. if you add, remove, or replace any hardware device that is in the GUID, the computer will get a new GUId. This probably only applies to people who want to escape their OEM product key. I think the sound card, graphics card and network card and processor make up the guid. Just change one of them for a new number and you should be able to escape out of the OEM label.
If you want to "add" to the uefi to escape having to put in the key every time you install Windows, just make a system image of your windows and designate a hdd as a Backup location. Make the image after all the updates, drivers, and software have been installed. Making a second one at a network location is a good idea also. This will automate the entire installation. A send option is You can edit the wim file and stream in your product key. I think the file name is install.wim? not sure. I've heard of it but never tried. Third option is to make a deployment using an answer file. Basically the installation refers to a file that you prepare that holds all the information the Windows install needs. You can include other stuff as part of the install. The answer key is where you add the product key.