A cancellable event is now fired if WorldGuard disallows PvP.

src/main/java/com/sk89q/worldguard/bukkit/WorldGuardEntityListener.java

@@ -70,6 +70,7 @@
 import com.sk89q.worldguard.blacklist.events.BlockBreakBlacklistEvent;
 import com.sk89q.worldguard.blacklist.events.ItemUseBlacklistEvent;
 import com.sk89q.worldguard.protection.ApplicableRegionSet;
+import com.sk89q.worldguard.protection.events.DisallowedPVPEvent;
 import com.sk89q.worldguard.protection.flags.DefaultFlag;
 import com.sk89q.worldguard.protection.managers.RegionManager;
 
@@ -241,8 +242,7 @@ private void onEntityDamageByEntity(EntityDamageByEntityEvent event) {
 
                     if (!mgr.getApplicableRegions(pt).allows(DefaultFlag.PVP, localPlayer)
                             || !mgr.getApplicableRegions(pt2).allows(DefaultFlag.PVP, plugin.wrapPlayer((Player) attacker))) {
-                        ((Player) attacker).sendMessage(ChatColor.DARK_RED + "You are in a no-PvP area.");
-                        event.setCancelled(true);
+                        tryCancelPVPEvent((Player) attacker, player, event);
                         return;
                     }
                 }
@@ -278,8 +278,7 @@ private void onEntityDamageByEntity(EntityDamageByEntityEvent event) {
                         Vector pt2 = toVector(fireball.getShooter().getLocation());
                         if (!set.allows(DefaultFlag.PVP, localPlayer)
                                 || !mgr.getApplicableRegions(pt2).allows(DefaultFlag.PVP, plugin.wrapPlayer((Player) fireball.getShooter()))) {
-                            ((Player) fireball.getShooter()).sendMessage(ChatColor.DARK_RED + "You are in a no-PvP area.");
-                            event.setCancelled(true);
+                            tryCancelPVPEvent((Player) fireball.getShooter(), player, event);
                             return;
                         }
                     } else {
@@ -350,8 +349,7 @@ private void onEntityDamageByProjectile(EntityDamageByEntityEvent event) {
 
                     if (!mgr.getApplicableRegions(pt).allows(DefaultFlag.PVP, localPlayer)
                             || !mgr.getApplicableRegions(pt2).allows(DefaultFlag.PVP, plugin.wrapPlayer((Player) attacker))) {
-                        ((Player) attacker).sendMessage(ChatColor.DARK_RED + "You are in a no-PvP area.");
-                        event.setCancelled(true);
+                        tryCancelPVPEvent((Player) attacker, player, event);
                         return;
                     }
                 }
@@ -822,4 +820,23 @@ private boolean isInvincible(Player player) {
             return god;
         }
     }
+
+    /**
+     * Using a DisallowedPVPEvent, notifies other plugins that WorldGuard
+     * wants to cancel a PvP damage event.<br />
+     * If this event is not cancelled, the attacking player is notified that
+     * PvP is disabled and WorldGuard cancels the damage event.
+     *
+     * @param attackingPlayer The attacker
+     * @param defendingPlayer The defender
+     * @param event The event that caused WorldGuard to act
+     */
+    public void tryCancelPVPEvent(final Player attackingPlayer, final Player defendingPlayer, EntityDamageByEntityEvent event) {
+        final DisallowedPVPEvent disallowedPVPEvent = new DisallowedPVPEvent(attackingPlayer, defendingPlayer, event);
+        plugin.getServer().getPluginManager().callEvent(disallowedPVPEvent);
+        if (!disallowedPVPEvent.isCancelled()) {
+            attackingPlayer.sendMessage(ChatColor.DARK_RED + "You are in a no-PvP area.");
+            event.setCancelled(true);
+        }
+    }
 }

src/main/java/com/sk89q/worldguard/protection/events/DisallowedPVPEvent.java

@@ -0,0 +1,82 @@
+// $Id$
+/*
+ * WorldGuard
+ * Copyright (C) 2010 sk89q <http://www.sk89q.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldguard.protection.events;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.entity.EntityDamageByEntityEvent;
+
+/**
+ * This event is fired when PVP is disallowed between players due to a "pvp deny" flag.
+ * Cancelling this event allows the PVP in spite of this.
+ * 
+ * @author Score_Under
+ */
+public class DisallowedPVPEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private boolean cancelled = false;
+    private final Player attacker;
+    private final Player defender;
+    private final EntityDamageByEntityEvent event;
+
+    public DisallowedPVPEvent(final Player attacker, final Player defender, EntityDamageByEntityEvent event) {
+        this.attacker = attacker;
+        this.defender = defender;
+        this.event = event;
+    }
+
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    public void setCancelled(boolean cancelled) {
+        this.cancelled = cancelled;
+    }
+
+    /**
+     * @return the attacking player.
+     */
+    public Player getAttacker() {
+        return attacker;
+    }
+
+    /**
+     * @return the defending player.
+     */
+    public Player getDefender() {
+        return defender;
+    }
+
+    public EntityDamageByEntityEvent getCause() {
+        return event;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}