Any diode to protect an optocoupler with low voltage drop?
You don't need to worry about the reverse current rating of the diode. Unless the voltage is so high that it breaks down, the diode will not allow any current to flow.

Actually, a tiny amount of reverse leakage current may flow, but it will be so small you can ignore it.

That's for the reverse direction. The maximum possible current for the forward direction will - approximately - be your peak voltage (about 17V: \$12\cdot\sqrt{2}\$ V) divided by R1's resistance. You haven't specified the resistance but you should be able to calculate that.