I did a hike this past weekend up Washington and Jefferson and had a chance to go down the Jewell trail from the Gulfside Trail (the AT). The Jewell trail unofficial "stealth campsite" is occasionaly mentioned as a option to Lake of the Clouds and is about midway on the long open above treeline section of the AT between the south side of Eisenhower and Madison. This is not a formal tentsite, its basically is one small open area in the woods to the right with some smaller open spots spread out in the woods to the left nearby. There are no facilities and the water source would be from a small stream that meets the trail just about treeline. Note that this steam is drainage from Mt Clay and the AT so it may not be pristine. Other than this one flat area, the trail below slabs the side of the ridge for quite a distance before coming to any other spots. I would not recomend it to groups. If the trail is followed down it comes out at either the Cog Railroad base station (which has tourist type snackbar) if you take the upper spur or a parking lot for the Lake of the Clouds hut and an AMC shuttle stop idf you go straight. There is currently a bridge out on the trail to the AMC lot so you might as well go to the Cog base station.
I didnt have the GPS but based on the terrain on a amp its about a 870 feet drop in elevation and 3000 feet distance down off the AT. There is dense treecover in the area but it is quite exposed to westerly winds and may not be a good place to be in foul weather. The elevation drop is more than to the Perch (farther north on the ridge) but less distance. There is no signage but its pretty obvous as the trail above it is a dense treelined tunnel that you will be quite familiar with by that point in your trek across the whites.
Do note the Jewell trail is one of the major hiking routes up Mt Washington, there will be plenty of dayhiker traffic, and unfortunately it does get used on occasion as toilet stop by dayhikers so watch for "landmines"