Q. Can you fly an Asymmetrical Spinnaker which is tacked to a fitting on the centerline (without a spinnaker pole) behind the headstay, when the Headsail (Genoa/Jib) is attached to a whisker pole?
A. The answer to your question is found in RRS Rule 50.3(c).
A headsail may be sheeted or attached at it's clew to a spinnaker pole or a whisker pole, provided a spinnaker is not set.
This indicates a spinnaker meaning any spinnaker symmetrical or assymmetrical.
If the asymmetrical, tacked on the centre line aft of the forestay, is small and has a mid girth less than 75% of the foot, it is a genoa and must take any penalty incurred in the measurement of the LP.
If this is an NFS situation, only one headsail may be flown at any time. It must be attached to the forestay for its entire length, and be a legal genoa/jib.