It looks like you might be walking the wrong route. The CLR does not allow editing the metadata tables of any assemblies loaded into the current AppDomain. What are you trying to do?
Reflexil is an assembly editor and works like a plugin for Reflector. With Mono.Cecil, Reflexil is able to manipulate IL code and save modified assemblies to disk. REFLEXIL also supports on-the-fly C # and VB.NET Code Injection.