Again that sounds like a good idea. It would be nice to travel somewhere on the ACA routes as they tend to be the most tried and tested. 3 weeks is probably the maximum I could get off work and even this will be a push. Plotting my original journey on google maps has yielded some positive results in terms of a possible inn-to-inn journey but as you say, they are few and far between if I suffer mechanical failure etc.
Have you done much lightweight touring? I'm still unsure if I should go for a tripple chainset or not, some of the climbs can a bit too much and I have seen a lot of people injuring knees this way.
I haven't done lightweight touring, but did parts of the NT there on a fully loaded bike, using a classic touring triple. While there weren't many steep-steep grades on the route, those mountain grades are l--o--n--g. You'll be grinding away at a 5-7% grade for hours and hours. So having low gears helps.
There should be others here that did inn-to-inn tours along that section of the Northern Tier. You should check out Crazy Guy on a Bike. And maybe start a new thread here, because with the subject "Juneau-Seattle", those folks who did a credit card NT tour might not look here!