While singing daughter to sleep, I figured out a relatively clean solution to making chat-over-email not break Mailpile's scaling assumptions.
This means I've also figured out how to use Mailpile and its search engine for content that has a much higher update frequency than e-mail does.
Chat, Twitter... IRC, Fediverse? Maybe, maybe.
Obviously, I am not supposed to work on this right now. Other things are more urgent. But it's nice that the path towards such things is coming into focus.