You can try this software called Sleepless:
Powerbooks can run fine closed, just normally they're also attached to monitors and keyboards.
i took jirkas advice and just connected the adapter and it worked like a charm
Iíve tried a small DVI to VGA adapter and a DVI cable on my PowerBook G4. Neither option worked. Could you tell us more about the method you used to fake your computer to not go to sleep when operating in closed / clamshell mode without an external display? Iím desperate. Thanks.