Having thought about it some more, I'm wondering if this is a possible solution (assuming it's possible at all through FL hook)
Have human cargo take up extra space in non-liners and vice versa for normal cargo in liners. Justification here is requiring extra space for human comfort in the non-liners and having to fit cargo in small spaces (cabins etc) for liners.
Some price juggling would likely be required to keep liner owners making money.
