I think
@Mr. Drauer has the right idea. If you want a bunch of formal pictures (and they really are great) you'll need to wait in a bunch of lines, and probably do some before and some after dinner. (Does anyone know if they have a third session after the late seating?)
As long as you aren't more than 15 minutes late to dinner, I don't think it will be a problem, especially if you give them a heads up. You won't be the only one late to dinner that night, and as said above, pictures close during dinner, so its not like you can be very late anyway.
As for convincing your family, I suggest a multi-pronged approach. With our group 9, we told the kids (teenagers) that they didn't have to wait IN the line, as long as they stayed nearby and came when we got close to the front so as not to keep anyone waiting. They hung out by one of the portholes mostly and did whatever teens do (for younger kids maybe some activity books or something). For my brother-in-law, we pointed out that he had already paid his portion of the unlimited pictures, so this was the best way to get his money's worth. For me, my wife told me it was something that was VERY important to her. Nuff said. (She wanted a "Cinderella" shot with her on the upper part of the staircase in only one shoe, and me at the bottom of the staircase, on one knee, holding the other. It came out great.)
Oh, and they do come around at dinner too, so you will get pictures in your formals then also. When we saw them coming to our table, we took a moment to stand up together before the camera arrived so that we didn't have formal pictures with dinner plates.
Have a great time!