If I remeber my Adirondack history correctly, the route up the slide was actually part of the first trail ever cut to the top of Marcy. I'm pretty sure it Phelps who cut the trail and took clients up to Marcy via those slides.
I can't imagine that there ever would have been a trail from the Marcy-Haystack col to Panther Gorge. I only say this because it is so damn steep; not because I have any historical information. If there is an old abandoned trail, you can count me in for checking it out. I'm thinking we would start from Panther Gorge and head up to the col. I wouldn't want to descend it site unseen. I would suggest bring a climbing rope you can repel off of too if you do decide to check it out.
I would also be game for hiking Marcy via the path of the first trail cut up the mountain, including the slide from Panther Gorge. Again, I would bring a rope though, as the slide looks mossy, wet, and slick. Something makes me think the slide was cleaner back in the days, but I don't know for sure.
John
PS We are talking about doing this in the late spring to early fall, right?