Thanks for your suggestions!
I managed to rewrite mscorlib.dll via disassembling (ildasm.exe), manual instrumentation und reassembling (ilasm.exe). I also managed to convince Visual Studio 2010 to use the instrumented mscorlib.dll via a custom target framework profile.
What do you mean with: "Even if it were possible, you would have to rewrite all assemblies that you want to execute so that they have a reference to the rewritten mscorlib and not the "real" one." ?
I only want to use the instrumented mscorlib.dll for special C# projects I compile myself. I don't have to rewrite all assemblies in this case - right?