The old story on leaving bullets in moulds was not directly to prevent rust, but to prevent spiders and wasps from nesting in the cavities, which would draw and hold moisture in the mould.  All those nice vent lines in the block faces pass moisture into the interior of the mould as well as air out when molten lead is poured in.  The only way to prevent water vapor in the air from getting in is to limit the contact between air and steel by physical means (oil, grease, plating), prevention of condensation by keeping the moulds warmer than the room air, or by  chemical means (rust inhibitors in a closed container).  A lead bullet left in the cavity will slow down moisture, but will not prevent it, as it will come in through the vent lines and under the sprue plate.

David