How to use Managed Code (C#) to create an FTP Home Directory Provider that is based on the Remote Client IP Address

I recently had an interesting scenario that was presented to me by a customer: they had a business requirement where they needed to give the same username and password to a group of people, but they didn't want any two people to be able to see anyone else's files. This seemed like an unusual business requirement to me; the whole point of keeping users separate is one of the reasons why we added user isolation to the FTP service. With that in mind, my first suggestion was - of course - to rethink their business requirement, assign different usernames and passwords to everyone, and use FTP user isolation. But that wasn't going to work for them; their business requirement for giving out the same username and password could not be avoided. So I...