Regarding mean vs 99% etc. In this case all you care about is: did loading the script delay page rendering to an extent that it was perceptible to the user? It's basically a step function. 99% is appropriate in this case.

Want to do it yourself? This talk by Etsy a few weeks ago has some detail on how they did a similar thing:

http://www.slideshare.net/marcusbarczak/integrating-multiple...

Some links at the end of the talk. Infrastructure wise, I think you have to be prepared to pay for some expensive DNS before this kind of thing is viable.