ikshwaku,
some years ago (circa 1980's) I worked on a number of storage tanks for liquid sulfur, at refineries. They were always buried. Usually there was no barriers of any kind, just a buried concrete box with the top sticking out of the ground a foot or two.
Because they were concrete, they did get cracks. Some sulfur leached to the outside, but I guess it solidified when it cooled on the way through the crack and essentially healed itself. I ran into no regulatory issues with tank containing liquid sulfur. I'm not sure, but I think liquid sulfur is not deemed hazardous because it does not react with other stuff, like SO4, SO2 and the like.
If envronmental regulations are different now, you may have to provide containment. EPA regs and state regs should tell you what you need to do.
Regards,
chichuck