> [...] a fancy IRC with a new account that has to be set up - that's a hard sell when you're an org of 100+ employees [...]
Slack appears to support SAML SSO, with a list of supported providers and ability to roll a custom one (https://get.slack.help/hc/en-us/articles/205168057). Haven't tried/needed it, but wouldn't that care of this particular problem?
Can confirm - at my ultra-massive employer where we have a considerable number of very large Slack teams, we use SAML to authenticate Slack against our intranet ID and everything more or less works.