Right.

Rev 1 B&W G3 does NOT support booting from slave IDE devices because the on-board ROM for the controller does not understand how to access slave devices. OS X overcomes this because it loads a software "ROM" to handle the IDE controller.

Click to expand...

If it's not clear to the reader, what I should have said in my post above was that OS X overcomes the problem of no support for slave devices by overriding the built-in ROM once OS X has already booted. So you can use slave devices on the bus, but only after OS X is running. To make a device bootable, it must be available before OS X has booted, and therefore it must be configured as a master.
Peace,

Drew