Yep, that should work. If the extraInfo value is not an object or a simple value, it is converted to JSON notation.
If you want to do something different to seralize the data, you can write a custom appender that serializes the extraInfo itself.
Have a look at coldbox/system/logging/logEvent.cfc’s getExtraInfoAsString() method to see what happens to your extraInfo by default.
Thanks!
~Brad