I met a paddler today at Deception Pass who had designed and constructed a scuba system for kayaking. He put a small air tank in the day hatch compartment of his Chatham 16, secured it with inflatable bags so it wouldn't roll around, and ran an air hose out of a a custom hatch cover to a mouth piece held under the bungees on the deck. With this apparatus he could stay underwater a very long time. He made it to help his rolling practice, so he could take his time, concentrate on technique and not have to worry about coming up for air.
I've always said that when learning to roll it's important to remove all distractions so you can concentrate on technique. Do it in warm pool with a dive mask, so you're comfortable and can see, and don't have water running in your nose. The scuba adds the additional comfort of not having to worry about breathing.
Another trick I've heard about is to use a snorkel connected to a tube that runs down your sprayskirt into the cockpit. While capsized you can breathe the air inside the cockpit. Exhaled carbon dioxide will build up in the circuit, the amount depending on the length and diameter of the tubing. So if you are under a long time, you might find yourself having to breathe deeper to achieve adequate gas exchange. I've never tried it so I can't say much more about it. For paddlers who are just learning to roll it's probably too much to deal with. Try it at your own risk.