Sorry if folk already know this, but we shouldn't get Bluetooth keyboards/pointing devices mixed up with USB wireless (2.4GHz) ones.
I have a combo keyboard-with-trackball that comes with its own wireless USB dongle, and it works with our nettop PC (running Ubuntu). Moreover, I can use the wireless keyboard right from when I press the power button, even for interacting with the BIOS settings, so the USB dongle is clearly operating at a fundamental level in the hardware (independent of the OS). I get the idea that Bluetooth is an "OS-level" system, so BT is dependent on the OS managing it (i.e. you couldn't work with your BIOS settings via a BT keyboard).
Look forward to trying the wireless keyboard with the RasPi...
Raspberry Pi Zero W with ZeroStem USB board (2017) ("zero") - Raspbian
Raspberry Pi Model B (1st-gen - 2012) ("ryo-ohki") - Arch Linux ARM