That would be slick. I’ve been forcing the email to open in a new window, and encouraging users to close the window after the email sends. Perhaps there’s a way to have the window close automatically after send?

It looks to me like the “saveURL” parameter works with the email composer tool.   Your url redirect code would look somthing like this: 

/_ui/core/email/author/EmailAuthor?p2_lkid={{{$Model.Case.data.0.ContactId}}}&p3_mlktp=500&p3_lkid={{{$Model.Case.data.0.Id}}}&retURL=%2Fapex%2Fui%3Fpage%3DGBC_Case_View%2526id%253D{{$Model.Case.data.0.Id}}&saveURL=URLEncoded_Page_Destination

Note.  You have not URL encoded the full link for your RetUrl above,  which will cause things to break.  I’m guessing that the RetURL in your email page is not what you expect.   Take the address you want to send the user to (in a retURL or a saveURL and go to http://meyerweb.com/eric/tools/dencoder/  to get an encoded version.  

Hey Rob.  Thanks a ton.  I was missing the &saveURL piece

The script now looks like this:
/_ui/core/email/author/EmailAuthor?p2_lkid={{{$Model.Case.data.0.ContactId}}}&p3_mlktp=500&p3_lkid={{{$Model.Case.data.0.Id}}}&retURL=%2Fapex%2Fskuid__ui?page=GBC_Case_View%26id%3D{{$Model.Case.data.0.Id}}&saveURL=%2Fapex%2Fskuid__ui?page=GBC_Case_View%26id%3D{{$Model.Case.data.0.Id}}