The issue I would take with your suggestion of drawing straws is that one thing the rule is not ambiguous about is that EVERY pilot in a given class should be weighed before the same flight. Now, if I were at the Nats and found my self in second place behind a 4-round winner who, after round 4 was weighed and DQ'd, I would expect his first four rounds to stand as judged. Why? Because like you said, "you have to make some assumptions." If you start the contest before weighing the airplanes, you have made the assumption that the airplanes are legal. If that assumption was not made, nobody could fly.

I'm glad the CD puts the information out ahead of time for the Nats. He has to. Why? Because the rules aren't actually being followed to the letter, and AMA regs state any exception to the rules must be posted ahead of time. I'm in no way advocating weighing all aircraft before every flight, but if you start the contest with the assumption that and airplane is legal you don't get to turn around and say he wasn't just because at some point later he wasn't legal. There's no way to know what the weight was previously, and he was allowed to fly anyway, so the flights should stand. Weigh-in prior to contest gives you a baseline. Then spot-check those in contention prior to flying the finals.