The time has come and lightdm 1.2 is in Wheezy. There is no way but making it work. The good news is that I did it; I have a working system prepared as a customized Debian Live on a USB stick. The not so good news (but not so terribly bad either) is that the configuration is a little less elegant. However, there are only 2 small patches to lightdm to make it work. Actually, these are the same as the first 2 patches out of the 4 used in lightdm 1.0.
Here is the story: the 2 patches deal with removing unused lock files from /tmp (their presence influences the choice of the display number in subsequent runs of lightdm) and with adding an option to use the same vt for all X servers. This is the same as with lightdm 1.0. The difference is that the 2 remaining patches have not been applied. They don't apply at all, they are not even close, because the architecture of lightdm changed since 1.0 and the processes are started in a different way while the 2 remaining patches added an option to pass seat numbers to X servers and to greeters; both X servers and greeters are started as new processes. But there is a satisfying workaround: do not even try applying them, the trade-off being a less elegant configuration.
In the lightdm 1.0 solution there is one central multiseat.conf, which is used both by udev to name event interface nodes in /dev/input and by multiseat scripts to associate keyboards/mice with seats and to move X servers into their corresponding positions within one big Xinerama screen. The multiseat scripts were generic, so they needed to be told (in an invocation parameter) which seat they relate to; the solution was also nicely scalable - there was only one file, the multiseat.conf, which needed to be edited to specify the keyboards/mice matching to their seats. In the lightdm 1.2 solution the scripts cannot be told which seat they relate to (because lightdm is not patched to do so, for the reasons explained earlier), so some of them are copied, in as many instances as there are seats, and by their construction they relate to a fixed seat. The good thing is that the patches which have been applied are really simple, so it might be easier to make it to the lightdm mainline.
You can get the changes from here:
bzr diff -r1493 > multiseat.patch
and follow the build instruction from here.