Even though this only really affects a handful of people, it is a very relivant and important question. I would stick to the safe side by not OCing on a motorcycle without my CPL, but that's just me. I think it would be good to get an AG opinion on this, or something. I doubt there is a legal deffinition of "in". Who knows. Maybe. Look under the deffinitions in the driving laws, there may be something there.
It seems to me that the way the law is written, you could carry ON a motorcycle, but I bet you a police officer would misinterpret the law, and I bet you a judge would agree with the officer. You might be able to beat it in appeals, but who really wants to go that far?
Probably a better idea to get an AG opinion.