Implement osNpcGetOwner(key npc):key. This returns the owner for an 'owned' NPC, the npc's own key for an 'unowned' NPC and NULL_KEY is the input key was not an npc.

llGetOwnerKey() could also be extended but this does not allow one to distinguish between an unowned NPC and some other result (e.g. 'no such object' if NULL_KEY is the return.
Also, any future extensions to LSL functions by Linden Lab are unpredictable and OpenSim-specific extensions could clash.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/OSSL_Api.cs

@@ -2313,6 +2313,27 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
             }
         }
 
+        public LSL_Key osNpcGetOwner(LSL_Key npc)
+        {
+            CheckThreatLevel(ThreatLevel.None, "osNpcGetOwner");
+
+            INPCModule npcModule = World.RequestModuleInterface<INPCModule>();
+            if (npcModule != null)
+            {
+                UUID npcId;
+                if (UUID.TryParse(npc.m_string, out npcId))
+                {
+                    UUID owner = npcModule.GetOwner(npcId);
+                    if (owner != UUID.Zero)
+                        return new LSL_Key(owner.ToString());
+                    else
+                        return npc;
+                }
+            }
+
+            return new LSL_Key(UUID.Zero.ToString());
+        }
+
         public LSL_Vector osNpcGetPos(LSL_Key npc)
         {
             CheckThreatLevel(ThreatLevel.High, "osNpcGetPos");

OpenSim/Region/ScriptEngine/Shared/Api/Interface/IOSSL_Api.cs

@@ -180,6 +180,16 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api.Interfaces
         vector      osNpcGetPos(key npc);
         void        osNpcMoveTo(key npc, vector position);
         void        osNpcMoveToTarget(key npc, vector target, int options);
+
+        /// <summary>
+        /// Get the owner of the NPC
+        /// </summary>
+        /// <param name="npc"></param>
+        /// <returns>
+        /// The owner of the NPC for an owned NPC.  The NPC's agent id for an unowned NPC.  UUID.Zero if the key is not an npc.
+        /// </returns>
+        LSL_Key     osNpcGetOwner(key npc);
+
         rotation    osNpcGetRot(key npc);
         void        osNpcSetRot(LSL_Key npc, rotation rot);
         void        osNpcStopMoveToTarget(LSL_Key npc);

OpenSim/Region/ScriptEngine/Shared/Api/Runtime/OSSL_Stub.cs

@@ -513,6 +513,11 @@ namespace OpenSim.Region.ScriptEngine.Shared.ScriptBase
             m_OSSL_Functions.osNpcLoadAppearance(npc, notecard);
         }
 
+        public LSL_Key osNpcGetOwner(LSL_Key npc)
+        {
+            return m_OSSL_Functions.osNpcGetOwner(npc);
+        }
+
         public vector osNpcGetPos(LSL_Key npc)
         {
             return m_OSSL_Functions.osNpcGetPos(npc);