Well you could edit the local host file of every computer (Windows) in your office and add an entry with any domain name you want pointing to your internal ip....
always good for a nice laugh too...on trade deadline day we used to point the sporting sites...so say like britney spears are something....but that was back in the day with only a few computers..
Also it isnt that hard for whoever is in charge of your network to setup and internal domain for you....if they are using linux/apache, just add in an entry in the httpd.conf pointing to your ip address...or even the servers host file...same with a windows server, pretty easy and quick...you may be able to get their with
http://yourcomputername