Sadly it seems, unless an OEM explicitly enables it, Windows Phone 8.1 handsets will not support the HID protocol.
Oddly it seems all the underlying architecture is already in the OS, but apps are prevented from accessing the data being sent unless a manufacturer supported the HID protocol for specific devices and provided corresponding device-data to app developers.
This is presumably to allow OEMs to build devices with Barcode Scanners and built-in keyboards, but will not enable end users to add a bluetooth keyboard to their phablet.
See the documentation at MSDN here.
Do our readers agree that this is an odd omission when Windows Phone 8.1 has caught up with other mobile operating systems in nearly every other area?