I believe the functionality for this was added after 2.0 went out the door. It can be easily done in 3.0m* by editing loginsettings.properties in tomcat/webapps/pentaho/mantleLogin/loginsettings.properties.
Don't take my word as absolute on this, but I believe you can take the 3.0m* mantleLogin folder and drop in in 2.x.
-Mike