This is only a suggestion!
On my system, I've a dual monitor setup through a nvidia card, with two monitors. Nvidia provides TwinView, which gives one big screen. It's different from Xinerama, which offers two screens, like :0.0 and :0.1. Now with TwinView it's one big screen (:0.0) divided by the nvcard over the two monitors. X (and KDE) "think" it's just one screen, with dimensions 2560x1024. The problem now here is that the logingreeter appears in the center of the screen (x=50%). I can solve this by manually edit the xml file of the theme, by changing the relevant x positions, and change it from 50% to 75%.
This is sometimes a not so easy job. Some xml theme files are complicated, where the x has to be changed on several places. This is then a trial and error thing, which is not necessary.
The lates xml file I'm using (KDE 4.4beta, oxygen-air) is relativly easy, I had to change the x position only on 3 places.
Now my idea is to automate this. KDM offers the possibility to make a script run before the greeter appears.
This is very possible, but the big issue is here, how does this "adjusttheme script" know what x positions to change and which not. Second, the script has to decide when to change the x positions, and when not.
I've done already a script, which uses xwininfo to determine the size of the screen. Bigger than a certain value means TwinView.