I'm considering lacto souring a gallon of wort on the side using the lacto living on grain and then adding back to the batch when it is ready. This is definitely an intricate multi-step process that needs to be well thought out and planned if you don't want to toss the dice and take a chance. I'm of half a mind to let local microflora have their way but the other half of me says "hell no!" I like the idea of it being local, unique, and artisanal. I don't like leaving it up to chance. You know Vinnie doesn't leave it to chance. He knows what he's doing and takes it through the process. Maybe I'll just do two beers. Problem solved.
If you do decide to make a lacto starter using grain I highly recommend trying to minimize any oxygen pickup and if possible purge your container with CO2 to prevent any nasty off-flavors associated with this process (butyric, enteric, etc). A bad sour mash can smell like putrid, rancid, vomit, cheese, and crap. You definitely don't want to add that to your beer.