1. As of the next build, all links are considered external if they use [URL].
2. There is no way to reliably test links to see if they resolve as wiki-type forums without using something like CURL, which from a performance standpoint is a no-no for something as trivial as setting a CSS class.
Instead, the little-known feature of URL-flagging has been extended for the next build. You can force a class by setting the flag: FTP, IRC, NEWS, SECURE. You can also force the link to not gain the external class by setting the INTERNAL flag. All of these flags are translateable phrases.