I don't think you have to tell the BIOS about it
Just enter the correct device name in the HDToolbox (in UAE's Workbench) and save the information. When you reboot UAE it should be there.
I can't remember right now, but it's called something like uaehfs.device .. you can find it in the documentation somewhere.
You wouldn't happen to be Lee from Vintagecomputer, would you?
