It's going to depend on the type. Bluetooth requires a driver to connect, and there's no BT driver built into the BIOS. But some wireless keyboards/mice, like Logitech, use a different protocol to connect initially, then switch to bluetooth once the OS is booted. I would say a typical generic bluetooth-only device wouldn't be recognized outside of the OS.
This message was posted on behalf of Intel Corporation
I have done some research about this Bluetooth and I understand the Intel® NUC Kit NUC5i7RYH handles Bluetooth keyboard at BIOS level.
I tried the Logitech K810 keyboard and Logitech M557 mouse, install chipset utility, management engine and graphics drivers, also a BIOS update. I managed to work and change settings using the Bluetooth keyboard.