After talking with my local players and getting input from the outside community I've made sure that no one scheduled to compete in the Mace Championship I've come to a few conclusions:
1. Nobody has a problem with PvS being legal.
2. Most everyone competing has a copy.
3. Many of us want to use cards and or Mages from PvS.
Thus Paladin versus Siren WILL be legal for this event. I know of at least one Siren already being played in this event and one Paladin. They join the Straywood Beast Master, Priestess, and Skeletal Necromancer already confirmed.