Thia isn't overtone, but here's some very impressive livecoding in scheme/impromptu (http://impromptu.moso.com.au/), building up to a wonderful piano improvisation which has a very natural feel: http://vimeo.com/2433947
The author of Impromptu is now working on Extempore, which unlike Impromptu, is opensource, and runs on other operating systems other than OS X.