You raise a critical question. I used CommuniGate Pro for a number of years and they have settings to allow skins for specific domains.
Once you type
http://myotherdomain.com, it automatically switches to that domain's skin. That is what we're looking for here with Zimbra.
The former however, has been around for as long as say BlackBerry, so they've had time to sort out a lot of issues (just check out the number of admin pages and options to appreciate). But we need a modern interface with new thinking, hence the switch to Zimbra.
Another feature they have (which I have been able to sort out backend) is auto-signup. A simple option on an admin page permits auto-signup. Again, on a domain basis.
We need to add these to bugzilla. You should have the option to decide whether or not to give self-access to a domain. Perhaps we need to look at what the competition is offering and treat most features as "bugzilla" in our dear Zimbra.