Alan,
I had contracted a dev here to build something very similar to this for me last year, it works on java script which allows you to insert the responses into the wysiwyg & title of the message, thus allowing you to edit it before sending or allowing you to append another response after the first. It also worked on usernotes, reputation, and send email (admincp) in addition to posts, threads, and PMs.
Who knows- it may assist in developing this hack, I am sure the dev we worked with wouldn't have a problem with it, so long as he were acknowledged in the event some of his code ended up being used.
If you're interested in checking out the code, please PM me.