Hook for feed creation?

It depends what you want to do as for when you want to hook into the process. the_content_feed is fired for each item in your feed, so this probably isn’t the one you are after.

You could use pre_get_posts which fires just before WordPress queries the database:

add_action('pre_get_posts', 'catch_the_feed');

function catch_the_feed($query){

    if($query->is_main_query() && $query->is_feed()){
        //It's a feed!
        $variables = $query->query_vars;
    }
}

untested but $variables should be an array of registered query varibles and their value (usually populated from what was recieved in the url).

…or you could use template_redirect which is when WordPress has retrieved the posts and is ready to display them (or redirect the user to a feed template)