for releasing the "cloth" couldn't you use a spring constraint on the left and right corner? then you just set keys for the springs stiffness. let left one go to 0 at 100 and right one at frame 200.

edit: or you could directly set a key on "constraint" - a boolean that sets the constraint active or not. you have to go to channel box though to be able to set a key

and of course it would be a lot easier with ncloth since you won't need to create clusters, but can constraint directly to the cloth's verts.