Thank you kindly, RandomHuman for a patch that:

The admin_close_region method removes a region from the simulator without deleting it. 
The region can then be recreated by calling admin_create_region with the same UUID. 
There is also a change to admin_create_region to facilitate this.The reason I want 
to have this functionality is to make it possible to detach regions when they are 
idle and recreate them on demand through a web interface. It's probably doable 
using the existing methods by saving and loading oars, but it also doesn't seem 
like that should be necessary.

OpenSim/Region/Application/OpenSimBase.cs

@@ -503,6 +503,37 @@ namespace OpenSim
                 RemoveRegion(target, cleanUp);
         }
 
+        /// <summary>
+        /// Remove a region from the simulator without deleting it permanently.
+        /// </summary>
+        /// <param name="scene"></param>
+        /// <returns></returns>   
+        public void CloseRegion(Scene scene)
+        {
+            // only need to check this if we are not at the
+            // root level
+            if ((m_sceneManager.CurrentScene != null) &&
+                (m_sceneManager.CurrentScene.RegionInfo.RegionID == scene.RegionInfo.RegionID))
+            {
+                m_sceneManager.TrySetCurrentScene("..");
+            }
+
+            m_sceneManager.CloseScene(scene);
+
+        }
+        
+        /// <summary>
+        /// Remove a region from the simulator without deleting it permanently.
+        /// </summary>
+        /// <param name="name"></param>
+        /// <returns></returns>   
+        public void CloseRegion(string name)
+        {
+            Scene target;
+            if (m_sceneManager.TryGetScene(name, out target))
+                CloseRegion(target);
+        }
+        
         /// <summary>
         /// Create a scene and its initial base structures.
         /// </summary>