You won't be able to do it in php from within a block, but you could do it from concrete5 event handlers. Start buffering in a before render handler, then close buffering in a render complete handler and then process the captured output. It would involve catching most of the page, not just your area.

(I think this may be how some of the email scrambling addons do it - have not looked inside them for a long time, so cant be sure)

Another way to do it would be to modify the area output section of a theme to catch the area content and then process it before outputting.