There has been no evidence for or against Brock getting a cameo, hence this thread is speculation without evidence. Besides, the answer is simple; why do just a cameo instead of fixing up the character so even more people would want an official return?
I am the current leader of Save Our Voice Actors, a Pokemon fan group supporting the voice actors in the anime dubbed by 4Kids (seasons 1-8). Our goal is to provide The Pokemon Company international with encouraging feedback in support of returning the main cast members. If you're interested, please visit us at facebook.com/saveourvoiceactors