Taking the lid off on those machines is a reasonable amount of hassle i think, it does sound like a borked HD to me though.
One thing to try to try and get to the HD is to start it up in firewire disk mode. Connect the mac to another mac that is already turned on with a firewire lead. Start up the Powerbook with the T key held down, it should load the powerbook as an external firewire drive attached to the other mac. That said if DU won't see the HDD then this may not work either, but is another option.