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.

prebuild.xml

@@ -95,6 +95,7 @@
       <Reference name="OpenSim.Framework.Console"/>
       <Reference name="Nini.dll" />
       <Reference name="log4net.dll"/>
+      <Reference name="Mono.Addins.dll" />
       <Files>
         <Match pattern="*.cs" recurse="false"/>
       </Files>