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.