Mantis#1591. Thank you graciously, Sempuki for a patch that:

Currently module loading is done ad-hoc. I propose creating a simple 
loader class that leverages Mono.Addins (and perhaps the new .NET 
addins when they become available in mono). Attached is a basic 
patch for review that compiles into HEAD, but doesn't yet replace 
any existing ad-hoc loaders.

OpenSim/Region/Application/IApplicationPlugin.cs

@@ -26,15 +26,15 @@
  */
 
 using Mono.Addins;
+using OpenSim.Framework;
 
 [assembly : AddinRoot("OpenSim", "0.5")]
 
 namespace OpenSim
 {
     [TypeExtensionPoint("/OpenSim/Startup")]
-    public interface IApplicationPlugin
+    public interface IApplicationPlugin : IPlugin
     {
         void Initialise(OpenSimBase openSim);
-        void Close();
     }
 }