A 450-gram partly completed airplane probably has a wingspan of about 1 to 3 meters. If you have a partly completed plane whose wingspan is of about 1 to 3 meters, and it weighs 450 kg, it will never fly. You need to redesign it. Otherwise you have a rocket, not a plane (and there's no guarantee that it's a working rocket).
Maybe, in exceptional cases like the Gossamer Condor, a 450-gram partly completed plane might have a larger wingspan, like 30 meters. With enough thrust and wingspan, you can definitely get a plane weighing 450 kg to fly (or even one weighing much more, like your Boeing 747 example). But if your Gossamer Condor competitor is partly completed and weighs 450 kg, again, you are going to have to redesign it, because a bicyclist will not provide enough thrust to keep it airborne.
If your Boeing 747 assembly is running behind schedule, you probably cannot catch up by adding more ballast to the wings, or prioritizing ballast over riveting, even though lead or DU weights are quick to install, and rivets do not weigh very much. X-ray inspection doesn't add any weight to the airplane at all, but it's the only way to find certain cracks. If your Boeing 747 weighs 500 tonnes, that does not mean that it's a super-complete plane. It won't be able to take off.
So, no, weight is not a perfectly reasonable measurement of aircraft completion percentage.
Maybe, in exceptional cases like the Gossamer Condor, a 450-gram partly completed plane might have a larger wingspan, like 30 meters. With enough thrust and wingspan, you can definitely get a plane weighing 450 kg to fly (or even one weighing much more, like your Boeing 747 example). But if your Gossamer Condor competitor is partly completed and weighs 450 kg, again, you are going to have to redesign it, because a bicyclist will not provide enough thrust to keep it airborne.
If your Boeing 747 assembly is running behind schedule, you probably cannot catch up by adding more ballast to the wings, or prioritizing ballast over riveting, even though lead or DU weights are quick to install, and rivets do not weigh very much. X-ray inspection doesn't add any weight to the airplane at all, but it's the only way to find certain cracks. If your Boeing 747 weighs 500 tonnes, that does not mean that it's a super-complete plane. It won't be able to take off.
So, no, weight is not a perfectly reasonable measurement of aircraft completion percentage.