How do I override the taxonomy default content?
This information is added in taxonomy_term_page(). As you can see there, the page callback returns a renderable array, which means that you can easily replace it in a custom module by implementing hook_page_alter().

In that hook, check that you are on a taxonomy term page, look if the 'no_content' block exists, and if yes, override the #markup.

I suggest to use dpm() from devel.module to print $page so that you can more easily figure out  where exactly the page content is in that huge render array. Somewhere in system_main, if I remember correctly.