Lewis would surely think Open Theism, or Limited Forknowledge, was a joke. It is unbelieveable that any reader of either theological history, dogmatics or the biblical text could offer this as a serious theological discussion. Lewis being an Anglican had a high respect for the greatness of God and would consider this a silly proposal, as no serious theologian throughout the history of the Church would, or has, offered this opinion.
Open Theism is a child of rationalism, commonsense philosophy and the 18th century christian enlightenment. The commitment of those who espouse this philosophy is toward the freedom of man, or his emancipation from history, and a rational diestic formulation of God.
Many within the Open Theism camp will talk about the love of God and that God is love. But this is only God choosing to be so. Somehow this makes God virtuous, because He chose to be good. Yet he could equally have chosen to be bad. In fact He did really know what He was going to choose until he chose.
If love is a higher good than God, then love is God. God becomes just like us, only bigger and more powerful. This is the God of Mormonism, which also came out of the 18th century revivalism and christian enlightenment.