Ok, I’ve made progress, but still not working correctly. I upgraded nxserver to the latest version, so it now recognises the DisplayDefault key. I can now remote connect to my server using NoMachine, and I no longer receive the “No available sessions on this server” error message.
Instead, I am presented with a black screen. This seems to be tied to the XBMC instance running in full screen on DISPLAY :0. If I ssh into the server and kill the XBMC process, suddenly the remote NX window flashes and jumps to DISPLAY :1 where I am greeted with the Xfce4 desktop running there.
The question is, why is the XBMC process interfering with the NX window if it’s running in DISPLAY :0 and NX is supposed to be connecting to DISPLAY :1 ?