Idea

You can make a button on your toolbar that acts like a switch, disabling layout changes:

int mode = -1;
forobjecttreeunder(model()) {
	if (isclasstype(a, "Conveyor::Conveyor")) {
		if (mode == -1) {
			mode = !switch_protected(a, -1);
		}		
		switch_protected(a, mode);
	}
}

When I did this, I was still able to add and edit decision points and photo eyes, even when I couldn't move the conveyors.