I'm not in your guys' league and I hardly have a right to comment in this thread. But here I am doing it anyway.
First, it's totally fascinating to me that the acknowledged gurus of light backpacking are pondering what they could do with a 75lb pack! It's like you've come full-circle, so-to-speak.
Skurka, Dial, Jordan, Fornshell, Bell, Moak, Nisley, the list goes on. Conversing on the possibilities of what kind of a trip could be had if one was carrying the maximum cartilage-grinding load possible. Surprising!
I don't mean this to be in any way disparaging. I think that this concept is a natural extension of the power of lightweight backpacking: you can use lw backpacking to cover more miles in a given time, to have more fun in a given mileage, or to reach otherwise unreachable destinations. You guys have done the first two ad infinitum, and now you're exploring the third.
I found this thread this morning and I've read every word of it; I can't wait to read more of everyone's thoughts and specifically of Bill's experiences!
My only contribution to the brainstorm is regarding nutrition: If I were training for an epic unsupported through-hike in Alaska, I imagine myself doing two types of training:
Phase 1) Mental conditioning: Of course a person has to actually do a 780 mile hike to see whether they're able (and willing) mentally. To practice the toughness and pain tolerance needed, the techniques, etc. And to work out exactly how much and what to carry. This is what you're talking about now.
Phase 2) Physical conditioning: Over the next year I think would train my body. I would walk long distances with a heavy pack *but* I would come home to eat every night! Huge amounts of nutrient-dense food, fresh food, sports drink, sports conditioning powders, vitamins, etc. Foods that are not available out of a pack would build (and rebuild) my body faster and better than the instant mashed potatoes and snicker's bars that ld hikers live off of. There's also massage and physio at home to nip any nascent injuries in the bud before they become problems.
I think that Phase 2 would be critical for such a hike. Because trips like that break the body down.
I think that your physical training shouldn't consist solely of thru-hiking, because thru-hiking itself breaks the body down. I think you'd want to eat 2 excellent meals a day and sleep in a real bed at night during the "buildup" phase. Plenty of time for suffering later!
/my 2 cents CAD