To use a judder spring though, we'd need an extra friction plate to match the outer diameter of the judder spring, correct?
This is true. I think the outer-most clutch disk is different ID to fit the spring and spring seat.
Still baffled why they didn't use a judder spring originally. It probably has to do with the "easy pull" clutch only providing a very small displacement at the clutch pack, so by combining the two you end up requiring a very careful clutch adjustment and perhaps not complete disengagement. Who knows... just guessing.