I'm undoubtedly going to expose my own ignorance here, but.. how can you be in a spin (beyond maybe the initial 2-3 turns when you may be still travelling 'along'), when the aeroplane is 89-91 degrees nose down?
Surely once properly estabished, the overall vector of the plane is vertically down - so in order to be stalled, the wing needs to be at some significant degree from the vertical?
My only experience of an inverted spin was definately rotational, and relatively flat - the horizon line was in clear view. It wasn't in a pitts.