Pete, I think maybe he's talking about this: http://www.transambikerace.com/. It's a very informal "race", but I agree with you that a number of people have done these kind of distances when not in a race. I also agree with you that it's easy to resupply en route. Carrying 10 pounds of Clif Bars to save ten bucks would not be my cup of tea either.
Perhaps what the OP means by GWD is the GDMBR. Or maybe he means an ad hoc route through the Sierra Nevadas.
Now, a few tips about general delivery. Pick a medium-sized town that only has one post office, but a post office with generous open hours. Know what the hours are ahead of time, and Google the location of the post office ahead of time too. Pick a town that you won't risk getting to on one of the days with more limited hours, such as Saturday or Sunday. If you can plan well enough, pick a town that isn't your destination for the day, but rather some place you'll be in the middle of the day (when the post office is more likely to be open--but make sure the post office doesn't close for lunch, which they do in small towns). If you keep to a schedule, general delivery can work very well. Don't put too much faith in expected delivery times, but mail your packages with plenty of time to spare.