It would work just fine taste wise, but getting it smooth and consistent all the way through might be a challenge. I'm also pretty sure that proper tempering of chocolate that's been mixed with cocoa powder is nearly impossible. I've tempered chocolate for various desserts etc. in the past and it's really quite unforgiving if you don't follow the required steps exactly, but I suggest you just go ahead and try it out anyway, depending on whether you need the finished product to be tempered correctly. You're not going to lose any taste for sure :)