I think you can use the "conditional region" server behavior to only display that row when an administrator is logged in. Then use a second conditional region to use a hidden form element with the same name attribute instead when it isn't the administrator so that the value will be maintained.
(if you are using mysqli then you wouldn't need the hidden form element, you could just set the parameter to "WA_IGNORE" so that it will maintain the previous value when nothing is entered.)