Modifying final output

I’m trying to make an extension to search for email addresses and encode them a certain way, I’m aware that EE has it’s own methods of encoding or hiding with javascript, however our client has opted to use the reCaptcha method of hiding an e-mail address. I cannot for the life of me find a hook for an extension (or module/plugin if need be) to get and modify the final output of a front end page (similar to what show_full_control_panel_end would do for the back end). Right now I’ve modified the main index.php for testing and it works great, but I’d love to be able to attach some settings or options the client could change once we hand it off to them by way of an extension/module/plugin. If you could point me in the right direction as to how to attain this it would be great, thanks!

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