I did exactly this with a robust porter that I had. I added cherries to one gallon of it and then pitched the dregs from a Jolly Pumpkin Bier de Mars. It soured really fast, like 2-3 months, and turned out to be one of my best beers. it is definitely worth trying especially if your just using a gallon.
If God wanted us to filter our beer, he wouldn't have given us livers