This was a cool project - I'm impressed. Just wanted to point out that http://topicbox.com offers similar functionality. Topicbox won't interface with Slack but rather replaces it. Run by the folks who do Fastmail, it's a modification/upgrade of old mailing list functionality. If you can get your team to use it, you can do most of what you did with Slack over an email interface that also offers perpetual archiving. I checked it out as a solution for archiving important project emails late-arrival employees would need to read to understand a project. Topicbox and this are slightly different, but both solve the high-bandwidth/low-availability problem in a way.

These products that require always-on internet connectivity on robust connections really tire me out.