get window handle or screen DC of logon screen

my company is developing a remote-control system to manage our many hundreds of computers at client locations, and one of the things it needs to do is to allow the remote user to log off the current user and log on to another account. when I call GetDC(NULL), I get the screen DC, but it goes black when I log off or lock the console. is there a reliable way to get the actual screen image from the logon screen? the program running on the client computers is a service, running as the local system account. it's basically a replacement for logmein or gotomypc.

I believe that type of stuff uses either a mirror video driver and/or a Gina dll.

gg