This is what DOT says about foreign scuba tanks. This is from a letter from DOT about the use of foreign tanks in the US. For the right answer rather than hearsay contact
PHMSA - Hazardous Materials Information Center They have been prompt and helpfull with my questions.
"This Departments Hazardous Materials Regulations (HMR; 49 CFR part 171 - 180) do not prohibit a non DOT specification cylinder from being refilled and transported provided the charged cylinder is not offered for transportation in interstate commerce. The HMR are not applicable to the use of, recharging or transportation of scuba cylinders by private individuals for personal use. However whether or not a refiller will fill a cylinder is a matter within his own discretion and over which this Department has no control."