Or it might require having the remote X server listen for the connection on 6000+

As the OP wants ptkdb's window to be displayed on the local X server, it doesn't matter how the remote X server is set up.   Anyhow, letting the remote server establish a connection to the local machine's X server directly from the outside network typically isn't the best idea...

This is why I suggested to tunnel the ptkdb X requests through the ssh connection.  ssh knows how to handle "no-tcp" listening X servers, so it shouldn't matter how the local X server is set up. On the remote side ssh creates a socket (unix domain or tcp, depending on configuration), which the remote X clients can connect to  (this is what you get via xauth list $DISPLAY).  The respective requests are then forwarded to the local X server.  But for this to work, the remote X client application also needs to have a valid cookie, or else the local X server would reject the requests.  Hence the fiddling with xauth...

On my machines (Ubuntu "Oneiric Ocelot", both ends), setting the cookie didn't help; ...

Then you did it wrong :)  I'm doing this all the time (also on Ubuntu "Oneiric Ocelot"), and it works just fine.