We\'ve run into an interesting situation that needs solving, and my searches have turned up nill. I therefore appeal to the SO community for help.
The issue is this:
Most SFTP servers support SCP as well which can be a lot easier to find libraries for. You could even just call an existing client from your code like pscp included with PuTTY.
If the type of file you're working with is something simple like a text or XML file, you could even go so far as to write your own client/server implementation to manipulate the file using something like .NET Remoting or web services.