I have a bedslide 1000 in my truck bed, the assembly itself probably weighs 280lbs. I foresee some issues: The tolerances are very loose, to allow free movement on the rollers/slide assembly, even with it weighed down the cargo tray still "slams" up/down, side to side, and front to back (+/- 1/2 in.), the lock mechanisms are engaged. It's fairly noisy when on unpaved roads. If you are not parked on a level surface its difficult sliding the platform out, think gravity. Front of vehicle angled forward, difficult pulling slide out. Rear of vehicle angled back, difficult pushing slide back in. And I'm doing this at truck bed height, imagine doing overhead. The overall height of the assembly is 5", thats without the side rails. Then add cargo on top, things will be 18-20" above the roof line, center of gravity and all that. As an overhead awning it wouldn't provide too much coverage, the slide can only safely pull out 1/2 of its length. Example: I have an 8ft bed, tray only extends 4ft past the end of the bed. That's not much canopy coverage. Now I said safely, when the latch catches, you can mod it a bit and get an additional foot of extension but it gets sketchy if the weight of the cargo isn't all at the back.
The main problem is the weigh on the roof. Bedslide + RTT + 100lbs cargo. Probably pushing close to 500lbs. And with it extended its cantilevering a bunch of weight on the mounts/crossbars.
The concept is interesting and never seen it done. Maybe if you could fab something using aluminum or put sliding rails on a low profile roof top carrier instead.