IRC logging is a solved problem. Just have a bot log your channels and upload them somewhere. Then you can throw them in the same search index as your wiki and your mailing list archives.
Does it put them in my normal client? That UX is valuable.
Also, I forgot about private channels - Slack does the same thing for private channels, synchronizing permissions to join the channel with permissions to read and search the logs, which putting them in a shared site-wide search index wouldn't do.
