I tried to get the message and the following line of code works:
TextView dialogMessage = (TextView)dialogObject.findViewById(android.R.id.message); <
TextView dialogMessage = (TextView)dialogObject.findViewById(android.R.id.message);
I know the question mentions AlertDialog, but if you came here through a Google search and looking for the answer for a DialogFragment:
DialogFragment
getDialog().findViewById(android.R.id.title))