My POC chrome extension uses a JavaScript based AES routine to encrypt text with a given user and passphrase. What gets posted to a site is the encrypted text plus the user name plus a unique string that my extension recognizes.
If you have the extension installed it will scan the DOM for text that matches the known pattern and decrypt transparently given that you have the right username/password combo entered in the extensions settings.
All that is left is to share the username/password combo to whomever should view the text via a different channel. I lost interest and didn't extend this to public/private key pair but that is the next logical step. Also, the extension needs a nice UI and maybe some visual cues on the page as to which text was encrypted/decrypted.
Lastly, once the text is decrypted by the extension, theoretically the host site could query for the decrypted text and send it back via ajax. I couldn't find a great solution to this using chromes pathetic api's for extensions.
If anyone is interested in this, just message me.