It doesn't matter where the sensor is, in fact it makes more sense that it's as close to the passengers as possible as it's measuring the deceleration of the passenger compartment. By cutting a hole in the crumple zone there will be a change in the way the portion of car behind the crumple zones decelerates, potentially causing the airbag not to trigger in a situation where it otherwise should have... I wouldn't want to sign off on something like that
Note: not saying this doesn't also apply to repaired vehicles...