Good points Mrs. K - I forget that not all chickens live in happy, healthy conditions. So, OP do as Mrs K says and do not take anything that doesn't look healthy no matter how long you have to drive to see them. In the long run, it's just not worth it.
Personally, I wouldn't treat them for anything until I knew there was a reason to treat. Maybe bring the powder with you, inspect the birds when you pick them up to put them in their traveling boxes, and apply it then if needed. I don't think it would do any good just to put it in the boxes...