Hi there,
I don't think it's do-able in Pages by default.
However, if you have a copy of Pages Automator Actions* (third party actions) then you can do this quite easily with an Automator workflow as shown in the attached screen shot. NB. Find and Replace action only works in Word Processing mode, not Page Layout.
Alternatively, it's relatively simple to do it in AppleScript too.
Hope that's of some help!
r.
* In the interests of transparency, I'm the developer.