I was able to do this with a SUSE Linux server so I could VNC connect via PMP.
What I did was to add the SUSE Linux server as a Resource with the Windows Domain as the Resource type. This is an additional Resource besides the one used for Telnet/SSH.
So, under the Resources for Linux servers, I have one for Telnet/SSH and another one for Linux GUI (Windows Domain).
I selected Windows Domain as the Resource type because it allows for VNC connections.
Once the Resource is created, edit the Resource and enter the password for the VNC.
As long as your VNC is configured on your Linux machine with the same password and you open up the appropriate ports on the firewall, it should work perfectly.