You wont be able to add the port in the dns record so it would force them to use tableau.123company.com:8000
I would recommend using a reverse proxy in this situation. The Proxy would be a machine running a web server running on port 80 which proxies requests and sends them to Tableau Server. You could configure the dns record tableau.123companyname.com to point to the IP of the proxy. Then the proxy would be configured to forward traffic to tableau on port 8000.
Bring in your networking team and they will be able to help.
Information on configuring Tableau to work with a proxy can be found here: http://onlinehelp.tableau.com/current/server/en-us/help.htm#proxy.htm
Hope this helps.
Thank you! They will have to live with the :8000 at the end.