To determine that something is safe to take during pregnancy includes determining if it can contaminate your baby's blood. If it is not capable of crossing that barrier, it will not change based on amount of its use. While you're probably right about a lack of study on off label use, I don't see that one is necessary. If it doesn't contaminate the blood in a week, it won't contaminate the blood in 36 weeks. I'm no doctor and obviously have never been pregnant but this is to the best of my medical knowledge.
Zithro is prescribed off label as an anti inflammatory drug in place of ibuprophen because it doesn't have the same effect on the liver and it has the added benefits of its bacteria fighting attributes. I do not know if resistance can be built against zithro but since it isn't used as an antibiotic and is not commonly used as one in the treatment of cf, it should not matter. But you would be happy to know you are at a greatly reduced risk of contracting gonorrhea while on zithro. The preferred treatment of which is a one time shot of 1000 mg of zithro or a stout dose of amoxicillian. (No, I don't know that from first hand experience but it is a funny story how I learned that)