Well, my understanding of the 1 Km. limit is that it has to be inside of a 1 kilometre cube, centered at the origin (so 500 m. on each side of the origin).
If your tube is not straight and folds back and forth within that cube it should be OK, but I guess it depends on what you are doing.
Otherwise, as CBL mentioned, assemblies can now be more than the 1 Km limit in SW 2009. The parts are still limited by the 1Km. cube though.
Cheers.