You could do a sour mash like you were making a Berliner weisse - mash in the night before and let it sit overnight before mashing out and sparging. I would start by just doing a portion of the mash, maybe 5-20%, and then adding the rest of the grain and mashing normally the next day. That will help keep it from getting as sour as a Berliner weisse.
That's just one idea, you could also add acid, acid malt, do a full lacto/brett batch and blend it, add a bottle of lambic, whatever. Obviously some of the ideas are better than others
If you go with their method you could add some grains to speed up the souring of the Guinness.