I though you might be able to setup a workflow so that when the regarding is changed is changed, it checks to see if the direction was incoming and teh Regarding (Case) contains data, then assign it to your other queue, but that would cause a problem because you would catch too much.
Then I found out that the modifiedon and case's createdon are pretty close to each other... so you could do...
if Regarding (Case):Case contains data AND [E-mail:Direction equals [Incoming] AND [Regarding (Case):Created On on or after [ Minutes: 3 Before E-mail:Modified On] AND [Regarding (Case):Created On on or before [ Minutes: 3 After E-mail:Modified On]]]]
Then: Assign to XYZ
That will allow you to do it with workflow.