You could check out UltraVNC http://ultravnc.sourceforge.org - it is currently beta code, but one of the main interesting points is that the developers has provided a xdmcp wrapper.
It also provides some very interesting hooking/display drivers - with some efforts it should be possible to combine the drivers and the xdmcp wrapper and isolate one application in the mirror driver which would give the wanted functionality. mirrordrivers are allthough only supported on nt/2k/xp. Med venlig hilsen / Regards Franz Wolfhagen
