showDeletedComments.user.js
This script will show all comments in the chat history in a room for which you are an owner. I have added the a button that will appear on the GreaseMonkey menu allowing you to trigger showing newly deleted comments without needing to reload the room.
Adapted from http://meta.stackexchange.com/questions/246762/show-moderators-deleted-chat-messages-by-default-in-a-transcript