You wouldn’t want to mirror if you already have DreamHost hosting content on the “live” domain.
You would fully host a subdomain of one of your other domains. If you can, the same user for the “live” domain would make it easier.
When you are done with the development, determine what user will host it, and copy the site over to that user. Then update the “fully hosted” settings for the “live” domain to point to that directory. But if it is the same user, just use SFTP/shell to rename web directories, ie “live” -> “old”, “new” -> “live”.