Hmmm...I must have a bit of brain fade - this is something I've done before but I just can't remember how. Hopefully someone can please remind me!
I have a comp with a wizard holding a wand. He sweeps the wand around and magic particles (using CC Particle World) emit from the end of his wand. Not very original I know, but hey, that's what the punters want. Anyway, if I do an expression to link the producer pos.x and pos.y to a null parented to the end of the wand, obviously nothing happens, because the null's pos.x and pos.y do not change - its movement comes from its parenting. The arm only has rotation - no position change. Kinda like this:
The green cross represents the null, parented to the red arm which pivots around the black pivot.
CC Particle world's particles do not inherit movement from the layer it's on, only from movement of the source point (producer).