We feed it to Hazel every day. she has never had a problem with it. She does prefer it when there is more leaf than stem, though.
Chicory (Italian Dandelion) actually contains less calcium than dandelion, but more potassium.
BTW, newer studies show that restricting calcium is not necesarily a good idea, and the formation of kidney stones or grit is not necesarily related to the diet. I remember someone had a link to an article or something about this recently, from the latest veterinary conference, but I can't find it right now.
In humans at least, it has been shown that adding calcium to the diet prevents the absorption of oxalates in the body. When the oxalate and the calcium are present in the GI tract at the same time, they bind together and neither is absorbed. That's why it's a good idea to either drink milk with spinach, or add cheese to the dish.
I'm not sure whether the same holds true in rabbits, but I assume it's still a good idea to ballance oxalate containing foods with calcium containing foods.
I believe in a natural, ballanced diet anyway. It has sustained Hazel for 7 years now without problems, and she's remarkably healthy and problems free, according to her vet.