A new version of the Skyblock Animal Protection has just been uploaded to the server.
Villagers are now included in the list of the protected island animals in addition to cows, chickens, pigs and mooshroom cows. The original post that describes the protection in more detail can be found here.
Further more, splash potions of damage no longer damage any of the protected animals on islands other than your own. Thanks to X_e_p_h_o_s that helped find that it was possible to kill animals using these types of potions.
So if you have your animals locked in a pen with a protection sign at the door, nobody but your own island mates can harm them.