Electrolytic oxidation of sulfate to peroxodisulfate in water solution is part of the old method of a hydrogen peroxide production, no idea if it will happen in molten state as well - but perhaps it can.
To get sulfur dioxide you need to reduce sulfur from VI to IV, and reduction at anode doesn't sound plausible to me. On the other hand oxidation of oxygen and production of SO3
2SO42- -> 2SO3 + O2 + 4e-
seems to be reasonable idea.
But this is just shifting of the atoms on paper, reality can be completely different.