So, a few facts here (not my personal opinions).
1) 3rd and 4th place have already been eliminated. They are not allowed to come back to play the final match. As soon as the top 4 match concludes, the losers are eliminated. If the winner decides after the match is over that he doesn't want to play anymore, his following round opponent gets a bye. Therefore, the only way that the 3/4 players would be able to "take their place" would be if the would-be winner of the top 4 match conceded prior to the match being concluded.
2) So as soon as the first player in the final game says that he's not playing the final round, he has conceded the round, and the other player wins the tourney.
3) But more than likely, it's never an actual concession, it's a discussion of a concession between the players. Technically, this is collusion and is against the rules. If you're being a hard ***, you would DQ both players, and the winner would be the highest seeded player that was eliminated in the top 4. Note that there would still not be a game played between 3rd and 4th.
4) But in all honesty, you're not going to outright DQ them for discussing it. You're going to tell them they need to play it out and that you can't allow them to just skip the game. If they decide to set their ships up backwards, that's 100% absolutely allowed. Just realize there's no such thing as simultaneous flying. Therefore, whoever moves the final ship ends up winning the game - even if his maneuver would have flown his ship off the board - even if it's the same PS as the other player.
5) So therefore, the only true way to not play out the game would be to go to final salvo. This now exists on the final table. However, you must give the players the full 120minutes. Therefore, they would have to avoid combat for the full 120minutes to trigger the final salvo - you cannot do it after round 1. Oh, and since they cannot slow play, they actually need to play out the rounds. The best way to do this is to setup such that your ships are all doing 1 straight into each other. But note this does mean that you have to continuously set dials, reveal dials, and not go anywhere for the full 120 minutes to trigger final salvo. And yes - I've seen this happen (though it was round 6 not the finals).
So basically, the only real way to not play the game would be for the two to collude, and the TO to DQ them, for one of them to concede to the other, or for one of them to fly all of his ships off the board. There is no way to skip through the game to just final salvo or a chance cube to see who wins.
That all said, those are the rules, but there's nothing to say that strictly adhering to the rules is the most agreeable path for the TO.