Can anyone tell me if its possible to redeclare a C# class in IronPython? If I have a C# class, would I be able to monkey-patch it from IronPython?
You cannot monkey patch from IronPython.  IronPython treats all .NET classes just like CPython treats built-in types: they cannot be monkey patched.  IronRuby on the other hand does support this.
Redeclare .net classes in IronPython