Climbing Katahdin in winter would actually be the easy part. Here in New England, I think most of us winter hikers would consider a lengthy winter hike of the AT in ME/NH/VT to be an ideal team effort.
I’ve hiked our NH 4K’s in winter, and you cannot underestimate the benefit of a consolidated trail base. Even on hikes where we had 2+ feet of new snow, we benefited from having a trail base that had seen some traffic over the course of the winter. This will not be so for many of the trail miles, particularly away from 4K’s and 100 Highest peaks, as others have said.
That is, you cannot underestimate the time and energy required to track through unconsolidated snow, alone, mile after mile.
Trail finding can sometimes be difficult, as the blazes are white, and often they are buried. I have not found GPS to be as helpful as you might think in these situations. Any trail points you pick off a map may be off by 50 to 100 feet, much too far to actually relocate a trail by GPS alone.
I don’t think there are too many tougher NE winter hikers out there than Guy Waterman and Dan Allen.
It can be done, but I think it would be much more feasilble (and enjoyable) as a team effort. Count me in for a section or two.
If you haven’t come across it yet, here is a recent trip report from the Maine AT:
http://home.gwi.net/amcmaine/reports/report96.htm
Tramper Al