content filter (add_filter) for category description?

Question

Sure, hook it right onto category_description.

Here’s an example:

function category_desc_filter( $desc, $cat_id ) {
    $desc="Description: " . $desc;
    return $desc;
}
add_filter( 'category_description', 'category_desc_filter' );

Didn’t test it but based on the Codex it should work.

See the Codex for more info: http://codex.wordpress.org/Plugin_API/Filter_Reference#Category_and_Term_Filters