There aren't any syn free smoothie recipes as you have to syn fruit as soon as you mash, puree, cook, blend, smoothie it.
It's nothing to do with releasing sugars or anything like that, it's about changing the filling power of the fruit.
If you consider how many oranges it would take to make a glass of orange juice (estimate 6) you wouldn't be very full after drinking it. But if you tried to eat all 6 oranges, you are liekly to struggle to eat them all.
Even if you used the same fruit that you would eat in a day - 1 banana, 2 pears, 2 apples (I've no idea, I'm just guessing) you still wouldn't be as full from drinking them as you would by eating them individually.
Syns wise, you are looking at 4 syns per 100g of banana, 1.5 syns per 100g of berries, 2 syns per 100g pear, 2.5 syns per 100g apple - but the back of your Food Optimising book will give you other fruit syn values.
This rule also applies to any veg that you would think of putting into a smoothie. You can do whatever you want to veg and it will be free as long as it's not in a smoothie/drink. Once you start adding cucumber or whatever to a smoothie, it loses its free value.