I have seen several integrations between wordpress and symfony, but they are made with the goal of managins static pages content inside wordpress and symfony acting as the controller.
Wordpress plugin development api seems really basic and primitive and you have no way o code with the MVC pattern out of the box. So the big question is, what would be an starting point to create plugins in wordpress, and create admin sections in it, but using Symfony power to manage database access, form creation and validation, etc .....