Almost exactly what I needed – but…

This is quality. One can see that by looking at the code.

But there is one problem. It’ doesn’t work for custom taxonomies, although it aspires to do so.

The reason is that it checks get_taxonomies() in ‘init’ hook at default priority. At this stage not all custom taxonomies are registered yet, so I get only the builtin ones.

Could you please change line 28 in tctgo_plugin.class to
add_action('init', array(&$this, 'init'), 99 );

When I do that, it just works. Other high values, or doing it later than ‘init’ may also work.