It's because although the 'ignore_visibility' parameter had been set to true, eZ Publish still applies the filter as is defined in site.ini.[SiteAccessSettings].ShowHiddenNodes, so it have a higher priority and you content isn't retrieved.
Could you try to set this parameter to true inside a file override?
Philipp's good memory suggests that it defaults to TRUE for backwards compatibility reasons, because the "reverse_related_objects" fetch didn't used to have an "ignore_visibility" parameter. So I guess it's just one of those "gotchas"!
