Made a quick plugin and works fine:
Groups selected:
My group not selected:
But I got an error when viewing the item (topic, file, blog entry, etc) because it adds the tags as meta data (keywords):
if( \count( $tags ) )
{
\IPS\Output::i()->metaTags['keywords'] = implode( ', ', $tags );
}
So I'll have to workaround this. I'll try again in the weekend, probably.