First WP No External Links is a great plugin, thank you !
WP No External Links is somewhat like pretty link, but pretty link is an expensive plugin.
I have these suggestions:
4. Add an option for Include URL. Sometimes I just want to redirect some kinds of external URLs, and keep others .
5. Link separator enhancements. Like wordpress permalink format, Link separator can provide some variables ( like postid or pageid ) or some simple rules to produce more friendly link.
Expect for reply !!