I’m sorry to being negative on that one. Since initially I wanted something similar myself.
But I found it’s most obvious and reasonable to answer for an old message just via your MUA – regular email client. Than it brings the answer to current discussion and let you continue talking on the topic. So maybe we actually need a way to open a message in MUA.
Though the chat could win from giving user the context of the message, because sometimes I click info-button in chat to see full-message.