Isolate.Invoke for private static method with ref param

This feature was added since then.

Basically you should wrap the ref parameters with IBox<T> interface as you can see in teh example below:

public class Foo
{
    private static int DoSomething(ref int x)
    {
        x = 13;
        return 0;
    }
}

[TestMethod]
public void InvokingPrivateStaticMethodsWithRef()
{
    IBox<int> val1 = Args.Ref(10);
    Isolate.Invoke.Method<Foo>("DoSomething", val1);

    Assert.AreEqual(13, val1.Value);
}

See documentation for more info.