My knowledge is not at all vastly superior. More than anything else, I was responding to the article's assumption that you'd need/want to tansport this stuff in solid form. If you don't make that assumption, but instead assume the exact opposite, it doesn't seem to differ much from existing gas transport problems.
As for the rate of expansion, etc, that's probably a calculable property, and the bottom part of the pipe could be built with a diameter profile that accounts for it -- or you could drop a self-contained "expansion unit" down to the seabed, and run the pipe out of it.
As an example, look at this rocket engine:
That tube around the middle was used both to cool the nozzle and preheat the cryogenic propellant (LOX or hydrogen.... I can't recall which), and its diameter varies according to expansion of the stuff as it's heated, and thus maintains a constant pressure.
The basic approach would probably work for this stuff, too.