Jeremy,
A little late for somebody to reply to your question but I do have some experience of expansion joints during my service in the Royal Navy. HMS Amazon was the first in the class of 8 type 21 frigates designed by Vosper Thornycroft for the Royal navy. In 1975 she made her first long trip to the West Indies and met very bad weather. The stress on the all aluminium rigid superstructure caused some alarming cracking, as the flexing of the steel hull was too much for the rigid aluminium superstructure to cope with.
My ship, the second in the class, was about to leave Yarrow shipyard in Glasgow in pristine condition when a team of workers came on board and literally cut the superstructure through the middle through all the compartments amidships. A rubber joint was inserted between both halves.It looked like the joint connecting two railway carriages together except it was only about an inch wide and it was quite amazing to watch the movement in rough seas, as it could stretch much wider and then close completely It was said that you could crack nuts in the joint. Of course being a war ship, there was no point in trying to hide it. We did have ongoing problems for a while as some cables - in particular thick aerial cables -became overstretched in heavy seas.
I watched the documentary on the television for the first time last night on the subject of the expansion joints, when the divers filmed the joint on the Britanic to compare with the original ship's drawings. I would imagine that Harland and Wolfe used some very clever screening to hide the joint.
I hope that this is of some interest to you Jeremy. Dave.