It is illegal. The insurance companies will have the biggest problem with it. If I were a pilot I would jump at the chance. Remember, it is always easier to get forgiveness than permission. The owners of the aircraft are the ones out on a limb.
I can't tell you the exact regulations mostly because I don't want to look it up and really don't care. But, it does not pass the smell test and that is what matters. Remember, it does not matter what the law says, it matters what the interpreter thinks.