rss feed for a single user

[Carandiru]

hey there,
like the title says: I'm trying to customize the rss feeds by adding the option to get only the threads posted by a single user.
the url scheme looks like this: "external.php?type=RSS2&u=1"
u is for the user id.

I've also created a plugin, which hooks in on "external_query" in the external.php

PHP Code:

if(isset($_REQUEST["u"])) {
    $hook_query_fields  = " ,thread.postuserid ";
    $hook_query_where = " AND thread.postuserid = '".$_REQUEST["u"]."' ";
} 


I know, I should use the GPC filter instead of $_REQUEST, but anyway..
unfortunatly this script doesn't seem to work. I don't know if this has to do with the cache.
so what am I doing wrong?