are there any silly paper legal implications of taking someone half way around the globe when they have expressed a wish not to go
There are legal implications but they are hardly silly. False imprisonment is quite rightly a serious offence for which you can go to prison for a long time. The passenger would have expressed a "wish to go" when he checked in and then sat in his seat, but he can naturally withdraw that consent at any time, and depending on the circumstances it might be acceptable for him to do so.
At the end of the day the airline would need to demonstate that they had a lawful excuse to take the passenger against his newly declared wishes, and that would be decided by a court, and it would all depend on the facts of the case.
For example. If the aircraft is delayed, it is known the flight is going to be continued to be delayed, they hadn't shut the door, and the passenger only has carry on luggage it would be foolish for the crew not to let him off. They can huff and puff about insurance and security but they really don't override an individual's right to go about his lawful business. Other cases might have different outcomes, if they had started taxying the airline probably has lawful excuse to take him to the destination he had freely bought a ticket for, if the issue was forced a court would ultimately decide.