http://www.youtube.com/watch?v=zjm7vG2Ggnk
2:30, ivysaur latches on from above the stage. No telling from that how far above the edge you can be, but that tether was definitely from above the edge.
Edit: In fact, at 2:41 he manages to miss the edge but at an incredibly steep angle at which you'd never realistically need to tether. Oh how I wish I could test this.
Angles from the edge and the maximum height/distance you can be from the edge - anyone up to the challenge of documenting this? Seeing the exact range of angles available to tether the edge would be a really valuable contribution.
Edit edit:
http://www.youtube.com/watch?v=3UdDUHCEkuA 3:32, Ivysaur tethers from ages above the stage and quite far out too.