I am Catholic. My understanding of Free Will is likely quite different from yours but my understanding of "Open Theism" leads me to believe it to be rank heresy.
Roman Catholics and Reformed Protestants often agree about the totality of Original Sin and the absurdity of "open theism." I guess we just disagree over how salvation is accomplished by God.
I'm always happy to recall one of Mel Gibson's "Passion" interviews last year. He was asked if God created and controls everything.
His answer: "Yes, God created and controls everything. Me, my children, cars, the weather...everything."
Spoken like a Calvinist. 8~)