I assume you can use Reporter.log(...). This will store the messages in
the TestNG log and are available in the HTML reports.
ITestListener and IReporter are mainly for handling test execution
status (onTestFailure, etc) or to generate a final report at the end of
the execution (IReporter), so in this case they probably won't help.
More about all of these here:
http://testng.org/doc/documentation-main.html#logging
Cosmin