Can I boot my Power Mac system to the BeOS without the Mac OS?

Therefore, the normal process for booting the BeOS on a Power Mac involves booting the Mac OS. During the boot process, the Mac OS loads Be's "OS Chooser" extension. Loading the OS Chooser extension allows you to select an operating system to load. If you choose the BeOS, the Mac OS load process will stop, and the BeOS will take over the machine and begin to load.

So, the simple answer is, no, on Power Mac hardware, you cannot do away with the Mac OS, just minimize your usage of it (by loading the OS Chooser extension early in the boot sequence).