Right now, we need to whitelist an external link globally at MediaWiki:Spam-whitelist. The requirement to be listed on the whitelist page is that there is a page on which it would make sense to whitelist the external link (at least on en.WP).
It would make more sense if specific whitelisted webpages were in a slot of their own on the page which they effect. That way, we would not need to globally whitelist a particular external link.
This slot would be edited by editinterface permission only (so admin/interface admin).