https://github.com/sbabcoc/logback-testng
I just published a Logback appender that enables users to direct the Logback output from TestNG tests to the TestNG Reporter. This allows authors of TestNG tests and libraries to use Logback for their informational / diagnostic messages, and to have these messages included with the result records captured by TestNG for each test.
I've already opened an enhancement request to add support for custom Reporter implementations, but this should be relatively easy to implement.