Bootrunner Graphics Support

I have an EFI flashed Radeon HD 7970. The EFI supports standard monitor sizes like the 27" Cinema display, but it cannot deal with the larger sizes supported by the card.
I now have a Dell UW38 display running 3840x1600 and do not have a display until the regular driver loads. Oddly, the BIOS display for BootCamp is ok. But it makes it impossible to use the built-in boot manager (option key down.)

Does BootRunner have its own driver or does it use the EFI driver? Is the driver capable of handling 384x1600 displays?

Boot Runner runs after macOS boots up, over the Login Window, so it will work fine with your setup.