I found this: "Acids like lemon juice or cream of tartar are added to change the pH level of the egg foam, which stabilizes it. Too much acid will prevent the coagulation of the proteins during baking and alter the taste of the meringue, so measure accurately. "
I knew that you needed some acid to prevent the meringue from weeping -- that's the cream of tartar.
I didn't know what adding too much acid might do.
If you don't use deacidified (dutched) cocoa, you run the risk of the above, but I'm with Alix -- I'd use it and see.
Less is not more. More is more and more is fabulous.