A small plugin I use with the pathologic drupal module for handling links and images, but should work everywhere you want relative urls stored by tinymce.
For example: your development site lives at http://www.example.com/dev and your live site lives at http://www.examplde.com/. This plugin strips /dev/ from links created on the dev site and only stores the relative url. /dev/home.html becomes straight up home.html. This way the urls will be the same on both dev and live. The plugins does the same with img src.