You can try using x11VNC. They have some kind of experimental support for this: http://www.karlrunge.com/x11vnc/#FAQ-23
. Basically, they send the monitor a "DPMS off" signal. If someone happens to wake it up, it immediately launches a screen lock utility.
Also, don't know if this applies to you, but the guy from http://www.linuxquestions.org/questions/history/206664
is doing something similar. However, when he starts VNC remotely, it starts a new X session. So, the X session already running server-side is never affected, and people will not be able to see what you are doing...