The lock effect was concluded towards the end of the show; I hardly think the audience was thinking much about it during the intervening performance; they didn't know what would happen. When the spectator was called up to the stage at the end, she was very soon asked for three numbers. There was little opportunity for "embedded commands". There were really very few even remotely plausible explanations.
Azrael 5' s comment regarding the direction of rotation misses my point. Anyone who's used a real lock of that type would know that the dial has to be rotated alternately anticlockwise and clockwise (or vice versa, but the first two numbers of the combination are then different). Few members of the live audience would know that, but the much larger TV audience would contain many people who do. Also, when dialling a real combination, one needs to stop at the 3rd occurrence of the first number, the second occurrence of the 2nd number, and the first occurrence of the third number. Derren's instructions omitted those details as well.
For anyone familiar with the real lock, therefore, the correct combination definitely wasn't entered, so any explanation must lie with the lock itself. For anyone unfamiliar with the real lock, the possibility of a stooge being used, or some amazing last minute suggestion process would perhaps be considered, but both were rather implausible. Hence, once again, the lock itself is the only thing left to suspect.