With my PB is usually put it to sleep manually (from the Apple Menu), press a key on the keyboard, then quickly close the lid. This will cause the PowerBook not to use the built-in display. Once I've done that, I can open the lid back up so I don't have heat issues. The built-in display won't turn on until you either press F7 or unplug the external monitor. With this setup you don't need an extra keyboard or mouse.