I’d be more than happy to invite you to my own Mattermost instance, which does have GitHub integration set up, and create a private team for us to play around with if you’d like, as well as our GitHub organization so that I can provide a perse “overview” of the way I envision how it might look, and how the integration would be useful, etc.
If that sounds agreeable, I’ll try and fill the team I create with fake discussions and stuff before you join so you can see what it could end up as functionally in-production instead of a bunch of empty channels, etc.