On the Bikepacking.net "about" section, The DrunkCyclist has this to say:
"...ONE GREAT WORD We’re not sure of the genesis of the term “bikepacking,” but we at Adventure Cycling have been using it off and on since the beginning days of Bikecentennial–including utilizing it extensively to describe the riding on the Great Divide Mountain Bike Route. We know it goes back to at least May 1973, when Bikecentennial co-founder Dan Burden’s article titled “Bikepacking Across Alaska and Canada” appeared in National Geographic..."
I have scanned a copy of that article which is accessible here:
https://docs.google.com/file/d/0B58u7NAO-w8yM1lZMjdJRUxRLWs/edit?usp=sharingCheck it out.