No for OP's idea. Adding more and more people DO change the balance of powers. The difference between an a random idea and a good idea is on the fact you actually measure the consequences. Try again without breaking the game and maybe you'll have a cookie.
But !...
It sounds fair that we can be able to start without matchmaking, and add crew members ingame. So if you choose "4 players crew" and you're alone, you can choose matchmaking or go alone (but it's dangerous, take this). This way you keep 3 empty places on your ship for them to arrive, and if you can't do anything because you're alone, well, it's up to assume your choice. It's no different than being a real pirate waiting for his crew at the tavern, cuz you can't sail such a ship alone.
It seems possible. I don't think that the problem is related to the session filling as Rare seems to count the players in a same map by ship and not by people. It's a way to ensure the balance is respected, and that you will never meet a map of 100 one-man sloop or a map of 10 4-man galleons, since they have targeted an encounter every 15-30 minutes.