Each sub-domain in my multisite setup has its own domain mapped to it using the WordPress MU Domain Mapping plugin: http://ocaoimh.ie/wordpress-mu-domain-mapping/. My plugin assumes you are using the domain mapping plugin on your install. Basically one needs the ID of the site in order to tell the plugin which sub-domain to map which domain. When you type in mywebsite.co.za you actually see mywebsite.mymultisite.co.za – just one of your multisite sites that you’ve mapped.
WordPress assigns an ID to each site on your Multisite setup so it can find the site easily. This ID can also be used by plugins and is often hidden from users. But, where on earth do you find it? And when you do find the site ID you have to write it down as your domain mapping is done on another page. It would be great to have the ID available on the same page so I wrote this quick plugin.
Once installed the plugin adds a bit of jQuery to the WordPress MU Domain Mapping plugin. This fancy code allows the user to start typing the name of the sub-domain they wish to map to and upon selecting it, the site ID is inserted. You then just put in your domain and save. No searching for ID’s on another page.
Take a look at the WordPress.org Plugin directory here: http://wordpress.org/plugins/domain-mapping-site-id-autofill/