1)I drawed a sulfur-analogue of furoxan (nitrogens in place of nitrogen) using chemsketch....it has a wonderful density.
Sulfur formally is +6 here. +6 is possibly not very stable (hydrolizes). So steric protection is necessary.
2)ever thought of -S-S- bridges?
3) -C-SO2-C- in a (poly)cyclic-structure is very dense and provides sp2-hybridization(plane molecule=>higher densities).
It seems stable enough against moisture in a ring structure(steric protection?).