Hm. Five minutes of thought suggest to me that the following pseudocode would be optimal:

while(passed.time < 9.5seconds) {
outcome = eval(opponent.code(self.code))
if (outcome == "C") { c.count++ }
if (outcome == "D") { d.count++ }
}
if (c.count > d.count) {
output("C")
} else {
output("D")
}

Deterministic programs would always output the same thing so you know beforehand whether they'll cooperate or defect. For RNG-based programs you just bet on what's most likely.

I welcome big guns blowing large holes in this construction :-)

P.S. How do I indent in preformatted code?