I would probably do 1 syn per 20 calories as its so massively fatty.
28g of pork belly = 147 calories so 7.5 syns per ounce
92% fat, 0% carbs, 8% protein
EDIT: If you use the nutrition info here: Calories in Pork Belly and Nutrition Facts and run it throiugh the calculator it comes up at 6½ Syns per package/portion on Extra Easy (that's for 28g) so I was pretty close.
Yes thats what I did - that figure includes pork as free. Its going to give you the average for the whole pork belly - fatty and less fatty bits. I dont think you can really get round the fat, its just a fatty meat, obviously some bits are better than others but its still going to be pretty sinful.