Back in the day someone wrote a software called "Veraeusser"* to manage exactly the task of using a PowerBook with just an external monitor, leaving the internal display switched off. I know this for sure, as I checked a lot of different uses for the "dual head" graphics of my PB180 when it was new. Hunt down this piece of code and enjoy.

* The name "Veraeusser" is derived from the German word "veräußern", which means to ged rid of something / to sell something.