I don't think you can do it "officially" through Amazon, because if it were the case, I'm sure they would have this stated on their Kindle page, or at least there would be two prices on books, one for Whispernet and one for web d/l.
Pardoz's idea looks very doable, I'll try to get the necessary software and give it a run to see if I can make it work.
Well, if it does work, it looks like I am definitively buying a K2
.