Run GitLab Mattermost on the same custom domain as GitLab Pages?

We run GitLab EE (installed via Omnibus) on premise at git.megacorp.example, with GitLab Pages configured as subdomains to an external URL like megacorp.io. In addition to that I'd like to set up GitLab Mattermost, but utilize the same domain, using mattermost.megacorp.io as the Mattermost URL. Is there any way to do this, like excluding the mattermost. subdomain from those available to GitLab Pages config? Currently if activating Mattermost for the mentioned URL I get the regular 404 error page that GitLab Pages outputs on non-existent pages.