Not sure if this was known before, I just want to make sure other can play with this as well:
Today I wanted to find out if it is possible to get the onboard bluetooth working on my Asus X99 Deluxe board. I found this rather old guide, that seems to be for OS X Lion:
http://www.atpeaz.com/index.php/2012/using-unsupported-bluetooth-4-0-usb-dongle-with-os-x/
I adapted it for Yosemite and managed to get the onboard bluetooth working - to a degree.
The device is recognized by OS X, mouse and keyboard works. Airdrop works as well, although it takes quite some time to discover the other device. Trying a BT headphone put the system in an unstable state, so this is not a good idea.
Here is what I did:
1) Make a backup of /S/L/E/IOBluetoothFamily.kext
2) Copy IOBluetoothFamily.kext to somewhere for editing
3) Open package content, look for /Plugins/BroadcomBluetoothHostControllerUSBTransport.kext
4) Open that kext as well, look for Info.plist
5) In key IOKitPersonalities, search for entry "PID 6091 0x17CB VID 2821 0xB05", copy it and rename the copy as you like
6) In the copy, change idProduct to "6154"
7) Install modified kext to /S/L/E with the tool of your choice, repair permissions etc., reboot
Maybe someone can find out what is the matter with BT audio, would be great!
Have not checked out Handoff enablement...
Have fun!