This is something that is tied up on the application server install. When you install, oracle would pick up the hostname.domain as your url. It all depends on the default hostname on the server. I encountered somewhat a similar problem on iAS 10g install, and the solution was to set the default hostname properly. If you are very desperate to change the host information, my suggestion is to consult the Oracle. Metalink has a script to change the hostname information for iAS. You may want to take a look into it to see what are all the XML files you have to make the changes on to.

Good luck,
Sam