The site admin suggested I post this here, as I have in the Facebook group 'Overtone Singers Unite'. BTW, if you haven't already become a member of that group, I recommend it.
The subject of this blog is collaboration online. With overtones of course. The site I use is kompoz.com
You can find almost anything here in terms of style. There are some quite accomplished producers, engineers, composers, etc. here, so don't worry about the quality. Of course, there are some wash outs, but if you dig around, you can find some real gems, who welcome anything and everything to add to the mix.
I did this a while back, and without too much searching came across some tracks that really inspired me. I did the vocal tracks in about a 3 hour time span. The mix was completed by the people who originated the idea. It was dedicated to the people affected by the Japan Tsunami, called 'Waves'.
It has been published here and is available for various levels of licenses.