Synaptic won't start in remote session, due to policykit

[robvh]

My LMDE is supposed to run headless, and X sessions beat VNC for responsiveness, so I use xdmcp to start a remote X session.
I understand this uses unencrypted traffic, but that issue is mitigated.

However, management tools like Synaptic and Update manager fail to start after prompting for a password.
After searching for 3 days I find a slew of discussion about policykit and remote sessions, like viewtopic.php?f=110&t=17630&p=106048&hilit=policykit+remote#p106048, and several attempts at workaround. But I see no real fix that "work for everyone."
Also https://bugs.launchpad.net/ubuntu/+sour ... bug/614291 with no apparent response.

After having fought Ubuntu versions long and hard to get xdmcp running reliably, and seeing that I got it running and functioning in LMDE 10 minutes after I first installed it (Easter weekend), I aim to stick with LMDE. If only I can run the machine headless, and in a way that fits the mojo of LMDE.

So: are there recommendations how to make policykit recognize remote sessions, that will work?

[robvh]

Here is someone who wrote a patch to integrate consolekit support into xdm: https://bbs.archlinux.org/viewtopic.php?id=112689
The patch seems to have been incorporated into xdm 1.1.11 in a modified form (https://bugs.archlinux.org/task/22671).
But not in gdm3