Those are all excellent posts. I measured it and there is only roughly 4.1" of clearance from the top of where a receiver would be to the bottom of the spare (33" Adventure spare). The Z-bar or drop receivers are not cheap (~100) can be found at this link:
http://www.hitchcorner.com/ball_mounts.htm#drop_down
That would allow you to use a Thule expressway 996XT which comes with a nice locking mechanisms. But by that's as much (or more) than the GM OEM rack. Maybe its worth a gamble.
Roof rack? How tall are you? I know that it may be better in some ways, but I park in a garage for work and besides what if I have to haul my kayak at the same time. For me, when I am done mountain biking, I can barely tip the bike back upright, let alone lift it up to the roof. Though I see that they make tire steps. What about a fold out hitch steps for big lazy dogs?
Its a funny thing that one of the reasons I started to look for a new truck-like vehicle is for the hitch (for a bike hitch rack like my friend's), but its still one helluva cool ride.