In newer versions the LVClassLibrary class has a Class Data Control property which returns a reference to the private control VI, which you can then parse through like any regular VI. Additionally, if the class is in memory, you should be able to open a VI reference to MyClass.lvclass:MyClass.ctl to accomplish the same thing.
That said, I have no idea what happens if you try to edit that control. I assume it should work, but that's just a guess.
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