Mmmhhhh... I lost HID Proxy Mode when pairing my Apple Wireless Keyboard into Windows... So I guess the same thing could have happened to you.
Or, it could be also that your keyboard doesn't support HID Proxy Mode (unlikely, but who knows...)
I did have to go back into OS X, unpair/pair keyboard, unplug/replug the BT receiver, then I got my HID Proxy Mode back.
By the way, find yourself an USB Keyboard before doing so, it will be much easier if you can't get your wireless keyboard to work during a certain amount of time.
First thing first, verify inside the property windows of your BT Keyboard Device, under Device Tab, if HID Driver is checked.
If so, try to uncheck it, and unpair/pair your keyboard.
If pairing is successful, then, try to unplug/replug the BT receiver (to reinitialize its state).
Then reboot, and, while your computer is displaying the BIOS page, hit a key, doesn't matter which key, just hit
(you can also try to hit several different keys if you wish
)
If HID Proxy Mode is properly set by BT receiver at startup, and if your keyboard does support it, you should see, just after having stroke the key, a light on your keyboard (if it does have one) indicating that a connection attempt is being made to a BT receiver.
Then, you should be able to hit the "Del" key or whatever other key to enter your BIOS settings.
The problem is the time needed to establish connection in HID Proxy Mode.
I noticed that right after my BIOS page is displayed is the exact moment when the pairing is really done, so that doesn't let me lot of time to hit the key to enter my BIOS
It's still possible though
BUT, since I also have other SATA devices to initialize at boot, right after my BIOS page is displayed, I have two others pages displaying (SATA devices initializing, 1. Intel RAID and 2. Marvell SATA), so that gives me much more time to establish proper HID Proxy connection AND to strike the right key (To enter BIOS, or Boot PopUp, or BIOS Flash Utility)