It would still come down to walking through the entire list of paragraphs (in all flows that appear on body pages), then through all table cells and also through any text appearing in anchored frames, although I assume that leaving the text in anchored frames (call-outs or sidehead notes) untouched might be acceptable.
I will have a look at the required loops and post a possible solution later today. I do not want to post code that I have not tested first. And this type of script might come in handy for some of my clients, too.
I am assuming that replacing every single character with an 'x' does the trick for you? It would not change the formatting or text flow.
Jang