A "host" (and potentially port?) field should be available for users to enter a host as separate from their JID's "host".
I, for example, use Google Apps for Domains. My JID is firstname.lastname@example.org, but mydomain.com does not resolve to Google's Jabber server. The server to authenticate with is "talk.google.com", and the JID is "email@example.com". This scenario should be supported by the plugin.