I think your best for this would be a plugin. One way to get it done would be to put a hook on GMail::Messages::insert_user. If you look at admin/GMail/Compose.pm around line 908, that's where it inserts the sent email.

What you can do is, if the email is set to be sent to the Sent folder, find out the parent of that email (and the location of it), and if it isn't the Inbox, change the destination of the email to that folder.

Adrian