From what I can tell (using Jenkins 1.518), the 'monitor external jobs' feature does not support usernames containing the @ (at sign) character.

This is a big problem for us, since we use LDAP auth, and all our usernames contain the @ sign.

When I use this sequence:
export JENKINS_HOME=https://foo@bar.com:secret@somebox.bar.com
java jar /somepath/jenkins-core*.jar "somejob" pwd

I get this exception:
java.net.MalformedURLException: For input string: "secret@somebox.bar.com"
at java.net.URL.<init>(URL.java:601)
at java.net.URL.<init>(URL.java:464)
at java.net.URL.<init>(URL.java:413)
at hudson.Main.remotePost(Main.java:93)
at hudson.Main.run(Main.java:74)
at hudson.Main.main(Main.java:56)

If there was a way to support multiple auth providers, then this would not be a big deal, since we could store our 'automation users' (the users we would use for monitoring external jobs) in Jenkins and use LDAP for the rest of the users. However, this does not appear to be an option.