Here, some processors require that birds be in stackable cages or crates. We used a processor that didn't have any particular rules and we used all wire cages to transport them there, which we picked up later, along with the processed birds. I think it would be a lot easier to unload them, already caged.
For those that take them loose in your vehicles, do you transfer them to the processors own cages/coops when you get there? I suppose it all varies by geographic region and possibly the size of the processor. I'm just curious.