Have you considered simply simulating keyboard input to the Pidgin window using C#?
You will need to find the Pidgin window programmatically and then set your status by sending simulated keyboard input to that window handle.
A good Microsoft example of how it was done with Calculator:
http://msdn.microsoft.com/en-us/library/ms171548.aspx
As I recall, Pidgin will set your status if you simply put the main window (buddy list) in focus, start typing, and then press Enter. Simulating this keyboard input should be quite straightforward.
A dirty solution (that future versions of Pidgin might break), yes, but certainly MUCH easier than writing a plug-in or making your own libpurple wrapper.