by Carlos Ors.
Hi again. I'm not an expert in moodle permission but afaik, the permission management of an activity should be managed regardless it is a normal tool or an external tool. In fact, if you don't have access to a tool due to a permission issue, its link shouldn't be accessible either. So, review the case of restricting access with a normal tool and repeat the same procedure with the external tool. If the behaviour was different, that would mean there is a bug in the external tool permission management.
Pherhaps you can find information in other Moodle forum section. I don't know.
Good luck.