Quote - The way I understand it Luxrender keeps working till you tell it to stop.

Correct.

Quote - You can’t make movies that way.

Sure you can. Look at the haltspp and halttime parameters in the film section.  You can stop a render after reaching a certain number of (average) samples per pixel, or you can stop after a specified amount of real time has elapsed.

Now, how you get an animation out of Poser and into Luxrender is something I'd like to know...