A great suggestion, and one that could be tested! The sand would have survived archaeologically, but the water most certainly would not. BUT... If the organic contents are not in direct contact with the outer pot itself, they won't soak into the pottery so an examination for residue might just prove useful, if only for a negative. There's no sign of charring or excessive heat on the stone or the concrete or the ceramic, so presumably everything got infilled with ash prior to the onset of the final searing pyroclastic flow...