I don’t know how useful this is, but I made a tool with EDK2 that lets you add NVRAM variables to your BIOS through the UEFI Shell. This does not modify/overwrite existing variables. The only usage I could think of is storing some data in your NVRAM that you can use for your own program, for example with the Windows library in C++ using the GetFirmwareEnvironmentVariableA() API.
Compiled EFI: Releases.