Ask HN: Instant Messages with a Public Record?

I’m a volunteer on a commission for the City of Austin. The commission is required to comply with Texas’s Open Meetings Act. Any communication by at least a quorum of commissioners must be publicly available.

The problem is that Slack and text messages don’t keep a public record. I could write a script that daily downloads Slack’s archive, converts it to HTML, and posts the HTML on a website. But I’d prefer a product, so citizens don’t have to trust me and my script.

Do you know of a service that does that? It doesn’t have to be Slack. Any instant messenger with good smartphone and web support would be acceptable. A way to record text messages (SMS or MMS) might also work.