Cuttlebone is actually very bioavailable. It's mostly calcium carbonate, which is the gold standard that they use to compare the bioavailability of other forms of calcium. It's best taken with food, because some stomach acid needs to be present to dissolve it. But cockatiels eat frequently, and even when they're not eating they have some food in the crop that's being sent down for digestion in a slow and steady process. So it's not a problem.
There are some forms of calcium that don't require stomach acid, like calcium citrate. But their bioavailability is limited in other ways, so it's not advantageous compared to calcium carbonate. It's good for humans who want to be able to swallow a pill and not eat any food with it, but that's not an issue for birds.
Harrisons high potency pellets have more than enough calcium, and if they're eating a lot of that you don't actually need an additional source of calcium. But it's good to have one available in case your hen wants more. If she already eats cuttlebone or mineral block, you don't need anything else.
If she wasn't eating pellets and refuses to eat cuttlebone or mineral block, it might be desirable to add some liquid calcium to the food. But if she's eating a lot of pellets you should NOT do this. It's possible to overdose on calcium, and quality pellet brands are already providing plenty of it.
P.S. I have an article on calcium at http://www.littlefeatheredbuddies.com/info/nutrition-calcium.html There's a section on bioavailability toward the end that also talks about the importance of Vitamin D. If your birds are eating Harrisons pellets they'll get plenty of D, but it's hard to provide this vitamin to birds that don't eat pellets because you can't get it from plant foods.
Another vitamin that's very important for breeding hens is Vitamin A, because it helps keep the mucous membranes in the reproductive tract moist and slippery. There's plenty of A in pellets, and vegetables provide it too. There's almost none in seeds and grains.