The only way to do this is to define your controller as a service and inject a custom logger with a custom channel.
Since the channels are created automatically there is currently no other way, but it's an interesting request and you're not the first, so I created an issue on MonologBundle to allow the definition of channels at the bundle configuration level. That way you could just fetch the proper logger from the controller using $this->get('monolog.logger.mychannel')
(which you can already do if the channel exists, but not if you want a custom channel for the controller that nothing else uses).
Update:
As of symfony/monolog-bundle 2.4.0 you can define additional channels as:
monolog:
channels: ["foo", "bar"]
Then you can retrieve it as $this->get('monolog.logger.mychannel')