A few things to ponder...
First of all, even if you were able to mount the rig to the crane, how would you manipulate it? I doubt that any off-the-shelf remote head has the ability to tilt and pan a fully-loaded Steadicam rig. Also, as you slide the camera off, you're going to need to slide on an exactly equal amount of weight, without disturbing the shot or letting the crane move around. Any error in that weight switchover could not only destroy the gear, but put people in harm's way. That certainly makes me a bit uneasy, especially since it's not a tried-and-true technique like an op stepping off a crane.
Another thing to consider is that the pause while you undock the rig and add the weight is going to significantly affect the momentum of the shot. Even if you worked with something smaller than a full Steadicam rig (as I saw several people on RedUser suggesting the Movi rig), you're still going to have to deal with the shift off and weight shifting on, and the necessary few seconds of time at the end of the shot as that switchover occurs.
Something to consider in this situation is to work around the limitations of the device. If you know you can't swap from crane to Steadicam easily, work in a way to not have to. One option is to hide an edit somewhere in there. An extra crossing the lens in the foreground is a common one, as is something architectural moving through the foreground of the frame (columns and doorways are great for this). As long as you're careful with continuity, it'll be a much easier way to not only alleviate the technical hurdles of a shot such as this, but also allow you to continue the pace of the move. Another option is to rather than bringing in a crane, build a ramp for you to walk down. It's a simple solution, and while it requires a good grip team to build it for you, it's much simpler and less expensive than trying to rig up a technocrane with a Steadicam sled on the end of it.
While it may not fit with the "one-shot" vision the director has, sometimes shots simply aren't achievable, and you must do what you can to get a shot that is doable, while sticking as close to the director's vision as possible.