since amino acids are photosensitive and tend toward an equilibrium isomer ratio

Well, amino acid racemization rates is high (under 1M years) at room temperature, but probably much lower at 3K or whatever the comet ambient temperature is, and the warm layers of the comet are regularly blown off, anyway. Not sure about the effects of X- and gamma rays on the racemization rate, but if it is anything significant, then yeah, little chance of finding anything but 50/50.

EDIT: actually, having looked a bit more, a deviation from equal ratio is quite common in extraterrestrial objects, and is an open problem, so the odds of finding something non-racemized are much higher than 1%. D'oh.