I have been involved in online music collaborations though my own site:www.musicollaborate.com
The thing to remember is that the person who is doing the work for you is the "Uploader" and you are the "downloader".
Because of the way most internet connections are configured, it is the uploading that takes the longest.
My collaborations have been restricted to passing mp3s forwards and backwards. Maybe in a few years the speed of connections will allow wav files to be send in a few minutes. Currently, if I was to send a full 24 bit wav file of a 6 minute song, it would take all night - it is possible, but I don't have that patience.www.musicollaborate.com