At one point I had exact contract references, too lazy to look right now but here is the basic idea:

If the scheduled DH had a backup, then in order for you to be covered by the 'unable to commute' policy so must your DD. If the original did not have a backup, then neither does your DD. Of course if your scheduled DH is late, cancels, diverts, etc, etc, you are pay protected and it is the company's responsibility to get you back on rotation, pay you, or drop the rotation and give it to somebody else. If you deviate and comply with the commuter policy then there as many outcomes as there are scenarios; however, I'm fairly certain you are at least protected from getting into any trouble.