Haha. All the cakes!!
It might be best to just make the frosting citrus-y. Chocolate is pretty overpowering, so even if you put some zest in (and stick to zest since oj is adding more liquids and that'd mess with the whole....chemistry magic unless you replace the milk or whatever with oj...) it might not come through too strong. But you could add in the zest or like candied orange zest to the batter. That's pretty easy to make yourself.
Candied Orange Recipe : Food Network Kitchen : Food Network <--- which would also make a nice topping after you frost.
So anyways, a citrus frosting would be fantastical, and that you can add both the zest and the oj. Ohhh.....with a sprig of mint on top or something..... Either way, this sounds like a fantastic thing and I bet your cake will turn out all.....chocolate orange magic
Let me know what you end up making!! And send me a slice