It can be a little complex, but I'm sure you can do it. 

- You can hide the login/register prompts by removing them from the skin, and creating a special skin (containing those tokens) that only registered users can see.

- Check the "Admin->User Accounts->User Settings" menu, there is an option called "Redirect After Login:"

The quickest way would be to edit the skin, and add some serverside login into the file. I quickly test on DNN5 with the default skin and it seemed to work, however the separator was still visible. You'll probably want to wrap those two controls in a serverside panel and toggle the panels visible state. Eitherway, the code snippet should get you going, paste it into the MinimalExtropy skin in DNN5. 

<script runat="server" language="vb">

    Sub Page_load(ByVal Sender As Object, ByVal E As EventArgs) Handles MyBase.Load
        If Entities.Users.UserController.GetCurrentUserInfo().UserID = -1 Then
            'should be an guest user... 
            dnnLOGIN.Visible = False
            dnnUSER.Visible = False
        End If
    End Sub

</script>