You are making it too hard.
If you all you want to do is to plugin your own router class then in app/config/parameters.yml add:
router.class: My\SymfonyBundle\Router
Basically, all of the framework classes can be overridden in this fashion. Take a look at vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Resouces/config/routing.xml
Don't use the getParent method. It's not doing what you think it is.
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Updated to response to a request for information about getParent.
getParent is documented here: http://symfony.com/doc/current/cookbook/bundles/inheritance.html
It lets you override a limited number of files from the parent bundle. It's really designed for tweaking 3rd party bundles. I got very confused trying to use it and tend to avoid it.
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One final note: I suggested adding router.class to parameters.yml because that was the easiest file to use. But it really should go in the parameters section of My\SymfonyBundle\Resources\config\services.yml assuming you are loading the services file.