So this seems rather unfortunate now but the first thought I had on reading your initial post was "wow, I wish he'd come and do some similar stuff for riot[1], but unless slack shut him down there's no chance." However lo and behold here we are, so I thought I might as well reach out. If you haven't heard of it, Riot can be described as "an open source slack" although I think that's unfair on it, it certainly lacks certain features that slack has (although with its current rate of development this gap is shrinking) but also can do things slack just can't.
Full disclaimer I'm not a developer on riot (or matrix[2] the open protocol it implements), just a happy user so I can't speak with true authority but I think they'd be happy for you to make these sorts of changes to riot.
So I invite you to come check it out, I hope you find it compelling. At the very least riot has a means of bridging to slack (along with irc, gitter and more) which you might find helpful in dealing with slack (I know I do).
Hope your future endeavors so smoother, I look forward to seeing whatever you create next.