I notice that Microsoft’s Project Mu has a PrintScreenLogger tool (Ctrl+PrtScn):
https://github.com/Microsoft/mu_plus/tree/release/201808/MsGraphicsPkg/PrintScreenLogger
and that it is already getting some forks:
https://github.com/vscpp/PrintScreenLogger
Before that, there was RU.EFI command line tool (F12):
https://firmwaresecurity.com/2016/09/09/ru-efi-updated-screen-snapshot-support/
and Nikolaj’s CrScreenShotDXE (LeftCtrl+LeftAlt+F12):
https://firmwaresecurity.com/2016/01/04/screenshot-taking-uefi-dxe-driver/
https://firmwaresecurity.com/2016/08/23/william-reviews-crscreenshotdxe/
And there are probably a few other options I’m not aware of, including by IBVs/ODMs.