screen resolution cannot be set with lightdm

Today, there are two sorts of computer monitors with digital input (mostly HDMI) available:

The ones designed to be used with computers. They report a list of supported resolutions where the highest resolution is the physical resolution of the panel. Since X11 always picks the highest resolution it can work with (or the first in the list), it usually picks the correct resolution matching the physical panel.

And then there are the ones designed for TV and video. These devices usually report a standard set of default video resolutions, such as 1920x1080, 1280x720, and so on, even if the listed resolutions (or the preferred) is higher than that of the panel. This causes X11/Ubuntu to pick a higher resolution than the panel has. This works with displaying movies, but not a computer desktop. Characters become unreadable, lines invisible, and everything is blurred. It is important to choose a resolution matching the physical resolution of the panel.

This can be easily with xrandr, e.g.
xrandr -s 1280x720

Earlier versions of gdm had a init directory, where shellscripts to be run could be put. Putting xrandr in a shell script in this directory simply fixed the problem.

But with lightdm I could not find a way to run such a command.

Thus, ubuntu with ligthdm cannot be used with displays of the second kind, i.e. all TV/Video monitors, although they use similar panels and could be used with Linux perfectly.

Where could I put this command to have it run by lightdm after starting the X server but before generating the login greeter?

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: lightdm 0.9.3-0ubuntu6
ProcVersionSignature: Ubuntu 3.0.0-9.13-generic 3.0.3
Uname: Linux 3.0.0-9-generic x86_64
Architecture: amd64
Date: Wed Aug 24 14:21:41 2011
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Alpha amd64 (20110802.1)
ProcEnviron:
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)