To expose a custom class in the editor, you would use the Serializable and rarely the SerializeField attributes. Public primitive types and Unity classes will serialize automatically and types which are given the Serializable attribute as well as arrays of such types. The System.Object should have this attribute and therefore in javascript, extending the System.Object (or one of the other know serializable types) will give the Seraializable attribute.

Non-typed containers and containers of typed containers will not serialize in and of themselves. To serialize a container of a non-serializing type, you would need to make the type serializable:

public class TransformArray extends System.Object {
    var array : Transform[];
}

var array : Transform[][]; //Will not serialize;
var array2 : TransformArray[]; //Will serialize;

To make a CustomEditor to edit a specific class is fairly simple (See the example in Editor).

Through the broken English, it almost reads as though you are suggesting dynamically building a class editor to create new struct classes. Is this what you are suggesting?

I think he means exposing one variable in the inspector, that by changing it will show expose other fields in the inspector... Like several operation modes for one class, each exposing different variables that relate to that mode.

And if that's the case SerializedProperty is a good solution for that. The example shows you can expose one private variable of the transform class. You can expose pretty much whatever you want, based on whatever you want.