I set it up out of curiosity, also aliened unity-greeter (which looks nicer). I would have packaged it but I didn’t even manage to compile it locally. I noticed that it doesn’t work with xdmcp (no matter wich greeter I use) unlike gdm, kdm and xdm. I think it is set up correctly - with XDMCPServer enabled and xserver-allow-tcp=true. It also listens to port 6000. The PAM configuration is the same as for the orther session managers. But I’m not able to log in remotely. I don’t see anything wrong in the log.
...
+0.01s] DEBUG: Launching X Server
+0.01s] DEBUG: Launching process 13114: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 vt7 -novtswitch
+0.01s] DEBUG: Waiting for ready signal from X server :0
+0.01s] DEBUG: Starting XDMCP server on UDP/IP port 6000
+0.01s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
+0.01s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
+1.53s] DEBUG: Got signal 10 from process 13114
+1.53s] DEBUG: Got signal from X server :0
+1.53s] DEBUG: Connecting to XServer :0
+1.53s] DEBUG: Starting greeter session
+1.53s] DEBUG: pam_start("lightdm", "lightdm") -> (0x65e180, 0)
+1.53s] DEBUG: Starting session unity-greeter as user lightdm
...