Presuming that a barrel processor accepts productivity modules; wouldn't this result in free barrels every time a liquid recipe is performed? I don't think that there would be any feasible way to prevent that; the only way that I could think to make it work is to have a different set of recipes for differing numbers of slotted productivity modules.
Perhaps have a 'productive' barrel processor with an alternative set of recipes, which requires a productivity module III to craft? Possibly a Mk II, III or IV, each built from the previous with the addition of another productivity module, to match an oil refinery, chemical plant or assembler.
