I know this thread is long-closed, but found it looking for "hidden text in Pages." Here's my work around:
I made a style (let's name it Hidden) with font size and line spacing smallest possible (that's .2 point and .02 point), no extra leading, text color white.
I also made a style that shows the text normally. When I want to hide the text, I apply the style Hidden.
It takes up some space, but it is a very tiny amount.