1) I wanted to build this ;)
2) Slack's IRC integration does not mark messages as read until you exit the IRC client. (Granted, mine doesn't mark them as read at all, but -- it _can_!)
My biggest issue is that Slack doesn't allow non-paying accounts (I forget what they call the tier - "guest" account or something) to use the IRC gateway.
There's one project I'm involved with that moved its support channel from Freenode to Slack. I used to idle in the channel 24/7 thanks to ZNC, and now I have to actually log in through the web interface every time I want to ask a question. It's their loss, too, because I'm also no longer as available to answer other users' questions.
If you run a software project, please, please, do not host your support channel on Slack. I don't want to have to request an invitation just to join the channel, and I don't want to have to either download and open the Slack app or use their web interface[0] just to ask a question or answer other people's questions.
[0] Their web interface has really terrible performance, and last I checked, their Linux "app" was just a webview around said web interface, with equally terrible performance.
Maybe they changed the rules since the last time you checked?
Not too hard and would be quite convenient (im not a slack user, but if i become one, id like that in case i log in the webui later..)