tag list made from specific category - wordpress

I think the method you've found it's the only way you can achieve what you're looking for. Maybe you can modify some lines, but the concept is right.

at the moment i don't think there's a way to filter tags as you would using a core wordpress function.

I did not get the code above to work my installation of WordPress. I did however manage to tweak it until it worked. Here is my tweak:

$catid = get_cat_ID(single_cat_title("",false));
$catobj = get_category($catid);
$catslug = $catobj->slug;
$all_tags_arr = array();
query_posts('category_name='.$catslug);
if (have_posts()) : while (have_posts()) : the_post();
    $posttags = get_the_tags();
    if ($posttags) {
        foreach($posttags as $tag) {
            $all_tags_arr[] = $tag -> term_id;
        }
    }
endwhile; endif; 

$tags_arr = array_unique($all_tags_arr);

$tagcloud_args = array(
    'include'   =>  implode(',',$tags_arr),
);

wp_tag_cloud( $tagcloud_args ); 
wp_reset_query();

Here is a much easier example.... Just change the category name and hey presto your done. The associated tags will print out in a list format.

<?php query_posts('category_name=html'); if (have_posts()) : while (have_posts()) : the_post();

    $posttags = get_the_tags();

    if ($posttags) {
        foreach($posttags as $tag) {
            $all_tags[] = $tag -> name;
        }
    }
    endwhile; endif; 

    //This snippet removes any duplicates.
    $tags_unique = array_unique($all_tags); 

    echo '<ul>';
        foreach($tags_unique as $unique) {
          echo  '<li>'.$unique.'</li>';
        }
    echo '</ul>';

    wp_reset_query();

?>