PlanetsCentral

I can see your point, but it's not easy to find a way out.
Actually PT-score is handling the problem of leaving players quite well. Though it prefers a defensive stance throughout the game.

What might solve your problem would be a score based on hullmass destroyed in battle. That seems, as far as I can tell, a good score for Northstar settings anyway.
However in that case it would still be important not to rush things because you can take on much bigger fleets later in the game.

Or maybe we set up a game with PBPs as victory condition. First player achieving a preset PBP value (like 10k) ends the game. The ranking would be according to the PBPs present. So the thing here is either use PBPs for building ships or for winning the game.
Since PBPs are invisible for foes this has the funny effect that game end might occour quite suddenly.