I'm not on your plan, so take what I say with a grain of salt, but if you really feel like you can't get through without cranberries, they aren't very sweet on their own, and you can make a sauce from scratch sweetened with Splenda. A whole cup of raw cranberries has 13 grams of carbohydrate, 5 of which are from fiber. That leaves 8 grams total, in a cup, of carbohydrate. If you sweetened with a non-caloric sweetener, and had a reasonable 2 tbsp serving, that's only about a single gram of carbohydrate.
In some cases, knowing you can't resist something and planning ahead to have a "closer to on plan" version can prevent major derailing.
Here's one recipe - it has a small amount of cornstarch to thicken - you could omit if you felt it necessary, but again, its a very small amount.
http://allrecipes.com//Recipe/homema...ce/Detail.aspx
I know IP has a strict protocol and is particularly limiting on carbs, but if depriving yourself of cranberry sauce and therefore diving into the leftover pie is at all a possibility, it might be worth making some sauce and having a small amount.