In Adium you can set your status really quickly by hitting the Status link at the top of your IM window. When you do this, it presents a list of available status, and of course one of them lets you create a custom status. But if you are often using the same custom status, you can have it entered into this menu directly.

Go to the Preferences in the menu, and then select the Status tab. This will list your custom preferences that are shown in the menu. To add one click on the [+] button below the list. It'll let you enter all the information for this new Status and when you are done you can hit the OK button it'll show up in the list.

If you then hit the Status link in the Adium messaging window, you'll see that your new status is shown and you can now select it with a single click.