I've read that too but cannot for the life of me detect any lactic or sour quality in Guinness. All I ever really taste in there is roasted malt and ... well, that's about all. I think Guinness is pretty one-dimensional as stouts go - maybe that explains its popularity.
Let's see, to sour 3 percent of 5 gallons you would need to sour 567.8 ml. Maybe you could collect 500 ml worth of wort from your next batch, let it sit in the open for a while until sour, then transfer into a pot or something and heat to about 170 degrees farenheight, then add it to your next Guinness clone. I would bet though, that much sour beer added to your recipe would be QUITE nasty and I would also imagine that it would taste nothing like Guinness but who knows?