Biscotti with chocolate drizzle in batter
You'd have to do it at the time you form the loaf for the first bake. 

One way would be to use a layer or two of chocolate sticks (as used in pain au chocolat but seemingly sold as "croissant sticks"). This would be very suitable chocolate as even melted it's not too runny. You could probably split them to get more, thinner sticks.  You could melt chocolate and allow it to cool in strips to make your own, which could be longer, thinner and less even. You'd need to have the dough mixed and then mix in whatever chocolate you use as little as possible.

A different approach would be to make a half batch of normal dough and a half batch of chocolate dough then combine those without mixing, as if making a marble cake. But this may not be what you're looking for.