Thanks @Tom, a couple of thoughts:
Maybe take a look at OpenNMS using Mattermost and Wordpress using Slack to see how Mattermost Team Edition could be used to reflect the identity of an organization with existing functionality.
If this is being implemented for a client, rather than being self-hosted, it seems important that the client know they're running Mattermost, so that if a security patch is ever required for their version they'll be able to apply the patch.