I want to know my very earliest/first chat post in a chatroom. Is there perhaps a way to search/view all my posts and sort the results?
It isn't documented, but using a bit of query param hacking, you can page through your chat entries, for example:
Just keep guessing larger and larger page numbers until you don't get any more results :)
It'd be nice to have an official way to page, though.
This doesn't truly answer the original question, but hopefully it points someone in the right direction.
The first thing you want to do is get to the "info"-page of the chatroom you want to get your first message from. As example: The Tavern on meta:
In the room-information panel you click on the "Search for messages containing"-Button. In the view showing up now you enter your username as demonstrated in following screenshot:
Then you search for something that's in almost every message you ever write.. like
I, order by newest, go to the last page you see, et voilà: