Yes, this was resolved with addition of the hook in Ingo 3.

Here is the hook for Horde 5 that needs to be added in horde/ingo/config
     public function transport_auth($driver)
     {
         switch ($driver) {
         case 'timsieved':
             return array(
                 'username' => $GLOBALS['registry']->getAuth(null),
                 'euser' => $GLOBALS['registry']->getAuth(null)
             );

         }

         // DEFAULT: Use hordeauth (identical to not defining hook at all).
         return true;
     }

I completely agree.  I was not asking the default behavior of INGO to 
be changed.  I was hoping that it could be updated to allow the 
default behavior to be overridden in the configuration for edge cases 
like I ran into.  The diff that I attached to the ticket maintains the 
default INGO behavior while allowing administrators to specify a 
different value if they so choose.

That's a bug in Dovecot then. The effective user is not (necessarily) 
the same like the authenticating users, that's why are two different 
user types.

I am referring to the credential's that are passed to the Net_Sieve 
object.  Specficially always using the horde username as the effective 
user.
From what I have been able to gather, the username that is used to 
authenticate is used as the effective user by Dovecot's managesieve 
server.   When authenticating against Dovecot's managesieve 
implementation the effective user needs to be set to null.  I have 
attached the perl script Dovecot provides for testing authentication.

I agree that setting the effective user to a static string or null 
does not make sense.  The intent of the patch I attached to the ticket 
was to allow the effective user to be set via  backends.local.php in 
edge cases and keeping the current behavior as the default.

I don't follow. What are you talking about?
That doesn't make sense, how is managesieve supposed to know for which 
user to accept the uploaded script, if you hardcode the effective user 
to a fixed value?

When trying to setup Ingo with my Dovecot Managesieve server I ran 
into an authentication issue.  Ingo was always including the Horde 
user credential when generating the base64 encoded plain 
authentication as the first field.  I need this field to be blank in 
order for Dovecot to properly authenticate.

I tracked the issue down to a line in ingo/lib/Transport/Timsievd.php. 
I went ahead an updated the file so it will use the "euser" parameter 
passed from backends.php or default to using the pre-existing call to 
Ingo::getUser(false).  I have attached a patch detailing the updates I 
made.