Okay, so, I have this working - not 100% seamlessly and not quite as it should, but it does work. I had do download Bluetooth Explorer for Mac OS 10.7 (linked from the Hideez documentation), and go into Utilities → HCI Controller Selection and manually specify the Cambridge Radio device:
Once that was done, I could go into the Bluetooth Setup Assistant and pair the Air 75 with the MacBook as normal (hold down Fn-1, Fn-2, or Fn-3 to select a Bluetooth profile; the left side light will blink blue when the keyboard is ready to connect, and the blinking will be faster when it’s ready to pair).
A stumbling block: I’ve punched in the below terminal command multiple times and the switch to the USB Bluetooth adapter never “sticks” across reboots. Option-clicking the Bluetooth menu item shows the address 00-1B-63-47-6A-4D, which comes up as an Apple device when I search. After I switch using Bluetooth Explorer, it lists 00-1A-7D-DA-71-03, which shows up as “cyber-blue(HK)Ltd.”
$ sudo nvram bluetoothHostControllerSwitchBehavior=always
So, I have that bit of troubleshooting left to finish. But at least I got it this far.