Maybe the solution, instead, is to create a simple browser plugin that accepts ciphertext and keys and decrypts blobs of text. It's ugly, but you could render it in place with some creative tagging of the blobs.

1. define a schema for the blob. recipient pubkey, signature, hash, etc

2. define a delineator or symbol of some sort to encapsulate the serialized blob so our plugin recognizes it

3. parse blob into readable text for the user

4. provide interface to manage/create keys/messages and load the blob to the clipboard for users to paste into whatever.

voila. please hold your applause, I'm going to need one of you to MVP this by the end of the week.