That's correct. Sorry - I wasn't paying full attention.
I would recommend that instead of setting and using Eic_AttDynamicAgentName, instead you set a Category on the email in your custom handler using ACD Specify Interaction Category.
If you set the Server Parameter AutoAddUsernameCategory with a value as described at http://help.inin.com/cic/mergedProjects/wh_ia/desktop/Optional_General_Server_Parameters.htm
then you just have to take the User ID parsed from the email, add whatever suffix you designate in the Server Parameter, then route to the ACD Workgroup in Attendant. That puts the email into ACD routing, but only allows it to go to the agent with the specified Category. The Categories are automatically added to the users when ACDServer starts, so you'll need to restart the ACDServer subsystem after adding the Server Parameter.