There are two ways I would approach this - either put them in secondary or make a tincture and dose the keg. I can't think of a beer I have made with spices in the boil that I thought was really good, even for just 5 minutes.
For secondary (or in the keg), put them in a bag (I would use about an ounce of whole peppercorns) and taste it daily until you like the flavor, then pull the bag out.
For a tincture, soak an ounce of peppercorns in six ounces of vodka for three days or so, then strain them out. Does that in the keg to taste.