Jim Peets told me that a lot of people feed their Stockblend 123 to pigs as they don't do a specific pig ration.
I don't know about any lamb feeds though - they'd have a different formulation. You could call and ask Jim Peets.
Otherwise, the OOD will relate to the vitamins moreso than any of the feed ingredients, so as long as it still looks and smells fresh I wouldn't be too bothered about feeding it beyond the OOD date (provided the meat from these pigs is for your own consumption, not for sale, of course), so if you mixed it at a relatively low proportion - say 1/5th lamb feed 4/5ths pig feed - I don't suppose it could do too much harm.
But personally I would call and ask Jim Peets.