[c#] How to call base.base.method()?

The answer (which I know is not what you're looking for) is:

class SpecialDerived : Base
{
    public override void Say()
    {
        Console.WriteLine("Called from Special Derived.");
        base.Say();
    }
}

The truth is, you only have direct interaction with the class you inherit from. Think of that class as a layer - providing as much or as little of it or its parent's functionality as it desires to its derived classes.

EDIT:

Your edit works, but I think I would use something like this:

class Derived : Base
{
    protected bool _useBaseSay = false;

    public override void Say()
    {
        if(this._useBaseSay)
            base.Say();
        else
            Console.WriteLine("Called from Derived");
    }
}

Of course, in a real implementation, you might do something more like this for extensibility and maintainability:

class Derived : Base
{
    protected enum Mode
    {
        Standard,
        BaseFunctionality,
        Verbose
        //etc
    }

    protected Mode Mode
    {
        get; set;
    }

    public override void Say()
    {
        if(this.Mode == Mode.BaseFunctionality)
            base.Say();
        else
            Console.WriteLine("Called from Derived");
    }
}

Then, derived classes can control their parents' state appropriately.