I'll try to remember to take a picture tonight for you of our prototype, it actually works out pretty well. Like you, we have a three stage design, but ours looks a little like:
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The center stage is attacked to the forks, and moves completely within the middle stage. The middle stage moves up to the top of the outer stage (with some overlap) and (not pictured here) the outer stage has a support beam at the top that goes behind the middle stage. It was actually pretty simple to hook up pulleys in the expected locations (the top and bottom horizontal support beams) for each stage - they just stick out the back a little instead of all being directly inline with the plane of the rack.