Yes. There's no need to refer to treaties, requalification of UN cylinders is covered by Federal Regs: Federal Register :: 180.207 Requirements for requalification of UN pressure receptacles.This definitely needs its own thread. Does a DOT certification facility have language to enter a cylinder inspection certification into a log book without a DOT number on the CE cylinder? Without a DOT number, even with any treaty language or agreement between Nations you still need a procedure to record values into a logbook. I doubt even DOT certification officials would know what to do. This is a long way off from solved.
Markings on UN cylinders are covered here, so the facility knows what it is looking at: Federal Register :: 178.71(q) Marking of UN refillable pressure receptacles