Hi! I wanted to underline this thing: to get into usb-download mode in the kindle fire, you have to short a pin on the motherboard to ground (shown in the picture attached). Since the paperwhite is so simple to open up it is possible that the same thing is necessary for the paperwhite, and they didn't provide a way to do it without disassembling. I don't have a Paperwhite (yet) but please someone look in this direction.
Additional info about kindle fire:
http://forum.xda-developers.com/show....php?t=1405052