Would phthalic anhydride work though? It is MUCH cheaper for SWIM. Besides, it can be re-generated.
Lets say someone did a batch with 1mole GAA and, say, 1.2 mole phthalic anhydride. The ac2O is distilled over and what's left in the flask is heated to 200C to melt. This will free H20 and regenerate the anhydride.
But really, back to the original question. What would happen if you refluxed GAA with H2SO4? We already know that GAA heated forms acO2 and H2O. Would the water NOT ionize with the H2SO4 present, shifting the equalibrium toward anhydride synthesis?