Access category within rss2_head hook?

I think you can just fetch it from the global query using get_queried_object:

function itunes_head() {
    if ( is_category() ) {
        $category = get_queried_object();

        if ( isset( $category ) ) {
            $acf_category = 'category_' . $category->term_id;

            $field = get_field( 'my_category_field', $acf_category );
        }
    }
}