As I mentioned earlier, znPlugin is a generic plug-in library that allows developers to add such functionality to their applications / libraries without forcing them to conform to any pre-defined interfaces.
Some competing libraries / frameworks are listed below:
MSINC .NET Plugins Library - A library, written in 2004, that does exactly what I am trying to do. I've been going through the documentation, and it provides some great ideas of how to reach my goal.
MSDN article - Food for thought on what needs to be taken into account when writing a robust library.
TechRepublic - Use C# and the .NET Framework to develop your own plugin architecture