I, too, would like this feature. It exists in MS Word; there's even a hotkey for it.
I have a suggested workaround that I have not tried; it depends on how smart Pages is about exporting formatting:
If Pages exports both style names and overrides, you could export to Word, open in Word, remove all formatting overrides, save it, then re-open it with Pages.
My favorite use for the feature in Word was when I was interviewing prospective technical writers. Since we were a Word shop, I asked them to submit their resume in Word. I then opened it in Word and pressed Ctrl+A Ctrl+Q Ctrl+Space. That selected everything and then removed all paragraph formatting overrides and all character style overrides. If the document did not change at all, I knew they had named all their styles which said they had a good handle on using the tool. If a resume was done all in Normal style with manual overrides everywhere, I was pretty sure that this was someone who would take a lot of training to work with styles properly. Most people were in between somewhere.