XMPP _is_ basically Jabber. So you should be able to login with an external Jabber/XMPP client, such as Pidgin, or Pandion (Windows), Exodus, etc. Once you have that worked out, you could likely integrate the nagios plugin (tho' I have not tried it).
One suggestion - use the resource of "zcs", so your web-client IM people and external-app people (e.g. Pidgin) can interoperate and see status changes, etc. So setting up the Pidgin account, use resource of "zcs"... and when adding people, I even tag zcs on the end, like this:
user@domain.com/zcs