Using your X and Y for points along the curve and (x0,y0,z0) for the "cone" vertex coordinates (with z0>0,) let (x,y,z) be coordinates of some point for which we wish to determine whether it lies inside the cone or not.
in = inpolygon(z0/(z0-z)*(x-x0)+x0,z0/(z0-z)*(y-y0)+y0,X,Y) & z<z0 & z>=0;
will be true if (x,y,z) is inside the cone. (I am assuming the cone stops at the x,y plane. Otherwise remove the z>=0 condition.)