Currently, messages in the chat beta that are considered great can be tagged with a star and make it to the list on the right. However, I do not see any way to "up-star" a message already in that list except for searching through the chatlog to locate the original message. It would be easier if one could just click directly on the star on the right pane to mark that message as great.
Source: raj at chat. Hover over the star to "star up"