iChat Screen Sharing

Although you're talking about file-sharing through the other screen and you controlling it you can't, but you could just use the built in file transfer in iChat to transfer files although it depends on what protocol you are using. To do this you can just drag the file to the text entry space below the chat and press enter of select it from one of the menus (I can't remember where as I'm not on a Mac right now). If you want the person you are talking to to share the file just ask them to do the same.

I hope I was helpful.

What about Back to my Mac? I haven't tried that yet, but can you drag from the host to the client to retrieve a file?