I don't think you'd get much of any nutrition from leaves cooked this way. You are better off going for their hearts, like you mentioned. Their hearts are loaded with carbohydrates.
If you had a pressure cooker and some sulfuric acid you might be able to break the leaves' cellulose down down into human-accessible sugars. This is "magic step" in non-starch based ethanol creation using things like switchgrass or other plant waste material.
Here is a patent that describes the process.
Sidenote: you'll need a pressure vessel resistant to sulfuric acid.
-Blast