I have no experience with apache handlers, so this might be way off, but in absence of a better suggestion here in this thread: Couldn't you just put the user parameter into r->args() even if it was a POST request ? And then access it in your code. Maybe you could even change the POST request completely into a GET by moving all the parameters from INPUT into $r and hacking the state in $r that says 'POST'

UPDATE: Found this in the CGI.pm docu which suggests that mixing is possible:

"It is possible for a script to receive CGI parameters in the URL as well as in the fill-out form by creating a form that POSTs to a URL containing a query string (a "?" mark followed by arguments). The param() method will always return the contents of the POSTed fill-out form, ignoring the URL's query string. To retrieve URL parameters, call the url_param() method. Use it in the same way as param(). The main difference is that it allows you to read the parameters, but not set them."