At the moment I am just carbing 5 gallons of water and keeping sugar free syrups on hand. But I too am interested in making some diet sodas using splenda. My understanding is that Splenda is sweeter then sugar.
says that SPLENDA« No Calorie Sweetener is available in granular form in a 9.7 oz. Baker's Bag, equivalent in sweetness to 5 lbs. of sugar.
Since your using two-liter bottles you could run some test batches to find just the right amount of splenda to add. Also since your using a carbonator cap you won't have to worry about adding any real sugar for the yeast. I would be interested in how it works out for you.