There is a lid closing switch in the front edge of the palmrest/keyboard, in the base of the Powerbook case, that usually triggers if the lid is closed, and this is probably what you'd need to ensure indicates to (fools/tricks) the system that the lid is shut (even though it would have been removed) so that the external display gets activated when you switch on/boot MorphOS - if I understand you correctly.

Which model of Powerbook is it anyway?

The LCD displays are quite easy to fix/remove/replace, and often not that expensive, especially if they are the low-resolution model.