Right. I think TinyMCE will just do that no matter what you do. Unless the dev's add some functionality to encode email addresses for us automatically (which would be a cool feature in my opinion). Maybe you could add it as a feature request to the trac?
Usually I tell my clients to not insert any plain mailto: links in the content. Instead, they should use something like address(at)domain.tld. I then use a jQuery plugin like this: http://www.ia-stud.hiof.no/~joakims/projects/defuscator/ to convert all the obfuscated mails back to "normal" mailto-links.