It depends on whether you’re going to a German opera or an Italian opera – and you left out that critical information. Frankly, we’re worried about you serving a mole, a Mexican dish, in either case!
Your question is challenging. In our online search, we really came across only one recipe categorized as an appetizer that includes cocoa, and it is a combination of chiles in a mole sauce. We doubt two moles will make a right. We also checked several books, and are coming up empty handed on savory appetizers that include some form of chocolate. The fact is, there are relatively few savory chocolate dishes. You could lightly dust some savory appetizers with a little cocoa – on a deviled egg, perhaps, in place of paprika. Beyond that, anything we’ve thought of just seems weird.
We do think, however, that dessert is not the place to be subtle if you want your guests to be at all conscious of your theme. After all, even a fairly educated palate may not pick out the chocolate in the mole sauce, and a little brown dust on an appetizer may not make a connection either. If it was us – and especially if it’s an Italian opera – we’d let the dessert sing!