It would be really cool as well as useful if Transmission could recognize when a file has been moved by the user. Transmission has a great set of group tools that enable users to save specific files in a given destination but I think it would be great if I could download a file and manually move it around my hard drive and Transmission would continue seeding the file no matter the location on my drive. This would also prevent transmission from thinking that the file was gone and automatically attempting to redownload it. Snow Leopard introduced a new set of filesystem APIs to make this possible.
I have looked into this - it's a cool idea, but since the actual code for getting the data is in libtransmission, which is C and multi-platform, it's not really feasible.