By crackling, do you just mean a piece of crispy fried skin served on its own, or are you looking for an effect like this one, where the skin on the pork belly is crispy?
The former is fairly easy - just cut the skin off beforehand and make crackling as normal while you cook the rest of the pork belly sous vide.
The latter - I'm of the mind that almost any effect is possible, but this one might take some engineering. I'm thinking that fully cooking sous vide, then cooling the whole thing to refrigerator temp (maybe even experiment with freezing it), then drying and scoring the skin of the belly, then rendering some of the subcutaneous fat skin-side-down in a not-very-hot pan like you were cooking duck breast, and then finally finishing under either a broiler or a blowtorch (keep it moving, always moving)..... MIGHT give you an effect like this. But really, it would probably require some experimentation. The above is just my best guess at how to proceed.
PS - the drying part I briefly glossed over will probably be very important. I've even read recommendations to use a blowdryer on the skin's surface to fully accomplish this - apparently, it enhanced the effect and final texture.
You've got me thinking I'd like to try this and see what effects I can come up with. Might do when I get some free time.
re: Spanish Willy
On the assumption that you're electing to go with separate pork and crackling and not the second alternative that Cowboyardee presents, here's how I did this last:
1. lightly scrape the fat side to remove a bit of the excess. You still need some of the fat or else it doesn't crackle.
2. lightly score the skin side with a box cutter; optional but I like the pattern and it helps you bite a piece off. Checkerboard pattern or diamond hatch - your choice.
3. Place a silpat or parchment paper on a roasting tray. Place the skin on the tray and sprinkle rock salt on the skin.
4. Cover the skin with a second silpat or parchment and then place a second tray on top. You'll need to lightly weigh this with something that can hold in the oven.
5. Roast for about 30 minutes (375F)
6. Remove upper tray, and jaccard the surface of the skin with either a really sharp fork, turkey lifters or needles.
7. Return to 400F oven and cook until crisp.