Interesting question, I myself got curious and decided to tackle it. I figured out a way to do what you are after. This is what I did. I vnc into 5901 for display:1 as usual or actually vnc over ssh connection. This is on KDE but should work for other gui's.
Once there open a terminal session on remote machine.
In it run the command ' export DISPLAY:0.0 '.
Now run any gui app with the & at the end.
Example:
xmms &
Now xmms will open up on display:0.
xmessage " This is a test " &
To display a message.
When done close the terminal session and now things work fine in your normal display:1.
You can also redirect an app back to your display but did not confirm it since the local machine was behind a firewall and did not wish to portforward back through it to try.
Found info here about it.
http://www.cpqlinux.com/remoteapps.html
Now for some more fun edit the remote server /etc/X11/xorg.conf file and then you can vnc to display:0 on 5900. So now you can watch what is going on on display:0 from local machine.
Add the Load "vnc" and Option "passwordFile" "/home/username/.vnc/passwd" in each section below.
Section "Module"
...
Load "vnc"
EndSection
...
Section "Screen"
...
Option "passwordFile" "/home/username/.vnc/passwd"
EndSection
Logout and restart X
Now you can ' vncviewer remoteIP:0 ' to show display:0
And also ' vncviewer remoteIP:2 ' to show display:2
Been a fun night. Learned a lot here. Hope this helps you out.
Brian1