I find the simplest way to do this is make the sketch invisible. The other step is to remove the java window as it creates a window. I will see if I can find some code on the old forums.

void setup(){
size(???,???);
.
.
frame.removeNotify();
}

This works but be careful as it entirely hides the sketch from the startbar and you can only end the sketch via the IDE or CTR-ALT-DEL