I've never heard of a mac server control panel, and nether have my technicians it would appear, even though we are somewhat "mac'ist"

... I couldn't imagine there being much involved from a developer point of view to create an application to simply provide an API or interface like cPanel to utilise in a web based script or otherwise for your users.
Thus potentially create your own with the aid of a development team as I am doubtful that you will something off the shelf for a server OS that isn't really used for the purposes of webhosting.