Dear Daniel,
I think the problem is not so much there's proof as there is two sides to every story. In the eyes of those who opposed him, they saw him as a tyrant. In the eyes of those who loved him, they saw him as a second Athanasius, defending the faith against a world of tyrants. The issue is not proof, but understanding the context of the issues at hand. St. Athanasius for instance was accused of many things that lead to a certain character assassination of him, but luckily for him, he was able to defend himself in a council. One of the most famous stories of the saint's shrewdness was having someone dress up like him and he look like a regular monk to confuse a prostitute who accused the saint of fathering her baby, where she ended up pointing at the man dressed up as patriarch who she thought was him.
However, in the fifth century, the situation did not end to satisfactorily appease both sides on the charges against St. Dioscorus. So I'm not sure if you'll be able to find a definitive answer to your question on this regard. Something else to keep in mind, St. Cyril, our common Church father and revered saint, was also called a Pharaoh by his enemies. That is not to say that I'm condemning anyone who calls St. Dioscorus a Pharaoh a Nestorian heretic, but I'm only mentioning this well-known reference as an example of showing that this is not the first time bishops of Alexandria received this derogatory "title", and probably even happened to bishops before St. Cyril.
