Sessions won't work well if you plan to have a load balanced infrastructure. Assigning IPs could be a maintenance hassle in the future. You could force the "Home" button to a certain URL per department by registry settings, but also a maintenance hassle unless you have a remote desktop configuration tool. If you have an HR system you can tap into, you could try passing the login ID on the computer through to the web server allowing you to automatically authenticate to the HR system and pull their department, then forward them on to their CMS.
Are you building your own CMS, or implementing a third party solution?
Secondly, why would you want the "Home" button on their browser to go to the CMS system. Would it not make more sense to make "Home" go to your new Intranet?