Hello,
Although there is no CMD_API_ADMIN_SETTINGS function in DA, you can still call CMD_ADMIN_SETTINGS with an API script. This would do everything for you with regards to the hostname.
The catch is that you must pass all variable that the form uses, which may be difficult to obtain since there is no API... but if the hostname change was only for new systems and rarely done, then you could just pick default values, along with the new hostname, and ns1/ns2 values, etc.. and submit it to CMD_ADMIN_SETTINGS with your script. The output would be html, so no likely readable, but there are not usually any errors with saving this form.
Note, that when changing the hostname, you must also ensure that the hostname resolves. I'd recommend adding a new zone into the dns (DNS Admin) with the full name of the hostname, eg: "domain = server.hostname.com", as this will also create an MX record which is needed for mail delivery from system accounts.
John