Override toString for classes you do not own in Groovy

I'm using a small Groovy application with Spring Social to consume a stream from Twitter. I now want to log every Tweet I receive, including some detail.

If the Tweet class would be inside my own code, i can easily do this to get a nice log output:

import groovy.transform.ToString

@ToString
class Tweet {
    // fields...
}

But since the class I want to log is inside Spring Social, I can't do that. Is there a "Groovy" way of overriding the toString method?

Adding a toString Closure to the metaClass does not really help, since I don't want to call toString explicitly at every log output.

org.springframework.social.twitter.api.Tweet.metaClass.toString = {
    "${delegate.field1}, ${delegate.field2}"
}

// still prints "received tweet [org.springframework.social.twitter.api.Tweet@637c1991]"
log.info("received tweet [{}]", tweet) 

// prints "received tweet [field1 field]"
log.info("received tweet [{}]", tweet.toString())