Imagine you have a site API that accepts data in the form of GET requests with parameters, or as POST requests (say, with standard url-encoded, &-separated POST data).
I would do it in the application, actually. It's still configurable at runtime, depending on your logger system, of course. For example, if you use Apache Log (log4j/cxx) you could configure a dedicated logger for such URLs and then configure it at runtime from an XML file.