I wonder if the cross section area of the superconductor is relevant. If it is not then it could conceivably be enclosed inside a carbon nanotube to hold its pressure. A carbon nanotube can hold about 620,000 atmospheres of pressure. Not quite 2.5 million but maybe another substance can be found that can superconduct at a lower pressure. Perhaps multi-walled tubes could be made. I'm sure someone has already thought about this though.