Wow -- this thread certainly did grow legs!
Thanks to all of you for your input -- I wasn't necessarily "encouraged" to take Lizzie but after weighing all of the admonitions for and against taking her with us I opted to adjust the route some and take her.
We'd been planning to do a section of the AT but in order to avoid traffic we took the suggestion of a forum member and visited the Pisgah National Forest just outside of Asheville. We were five in number, and my girlfriend is seven months pregnant, so we hiked in, set up camp, and day hiked from there to keep it easy on ourselves.
I think it was a perfect first trip for Lizzie: it was a low traffic area (except for the Boy Scouts who absolutely LOVED Lizzie) compared to the AT and we were able to keep mileage relatively low.
We did have her wear her new pack the whole time, but left it empty in the interest of her puppy bones. We wanted her to get used to wearing it while hiking about. We also split the days roughly in half off-leash and on, and when she was on leash she was just fine staying in a column.
It was most definitely an enlightening experience: we learned her strengths and also her many weaknesses as a wilderness dog. She's predisposed to staying with us, never wandering more than 20 yards or so, and excellent about preferring us to other hikers (though she did follow a group of boisterous women up another trail the first morning), and until the very last day was excellent about not approaching other hikers. She learned after the third foot-bridge to cross them very slowly and cautiously. She has, though, demonstrated what many have said is a classic hound's trait: a propensity for completely ignoring anything we say or do except when we leave. We definitely need to work on recall, and also on getting her not to eat poop.
Anyhow -- thanks again for all your thoughtful responses. Here's Lizzie at lunch on day one. She'd never walked that far in her puppy life.