I actually ended up hack-jobbing this in, if anyone else wants to play around:
if (!isset($common_params['tags'])) {
$common_params['tags'] = '';
}
$categories = get_the_category($post_id);
$output = '';
$separator = ',';
if ( ! empty( $categories ) ) {
foreach( $categories as $category ) {
if(!in_array($category->name,Array('excluded category 1','excluded category 2'))) {
$output .= $category->name . $separator;
}
}
trim( $output, $separator );
$common_params['tags'] = $output;
}