You are correct, the only way to change castor angle is to move the ball joint position. The only change you can effect with the mushrooms is to mount them in the wrong postion where thay can change the camber.
What the mushroom inserts are designed to do is to correct a design fault when using the Sierra upright. Basically when you draw a line through the top and bottom ball joints it is ideal to have the spindle axis on this line. The Sierra upright's isn't, hence the offset mushrooms, which rotates the hub to align the ball joints with the axle centre line better and increase the trail, which is what helps with self centring.
Hopefully that all makes sense
TK