Create a shortcut to Chrome and edit the path:
...somerandompath/chrome.exe --user-agent="User Agent String"
Thanks to http://knol.google.com/k/wardog-l/google-chrome-change-user-agent-to/187t5jnwvnu9d/6#
Some suggestions show how to modify the Chrome dll to further hard set the User Agent String. But this method allows you to keep a single unmodified install supporting multiple possible user agent strings.