Well, running a chiller and keeping a skylight on it is probably your best bet - easily the cheapest way to light a reef that size.
I'm hoping that by swapping in and out water during all daylight hours it should keep the tank cool enough to not need a chiller I'm just thinking of the awning to lessen the heat into the tank and if UV rays will be a problem for the acrylic coming in through the waterWell, running a chiller and keeping a skylight on it is probably your best bet - easily the cheapest way to light a reef that size.
The awning wouldn't be a complete block out of the sun just to reduce it as I'd be worried about coral bleaching with the water heating too high as they can be sensitive to heat and it would only be up a few hours a day, say 2:30-5 just to reduce the direct heat but it would still let light through (more like a shade cloth)You could keep an awning over it, but then you need artificial lighting. The corals need lighting to survive. Checkout reefkeeping's magazine with the previous tank of the months. There's one that's big and it's some weird number, like 1,532 gallons. It's pretty similar to what you want to do, except yours is 50 times the size
I don't know how it would work with a sleeve as I can't imagine how you could slide it in it must look like that but really be 2 plates bolted together to sandwich the acrylicthe large tanks at my local aquarium (several of which are roughly around the size you are talking of building) have the acrylic cemented in. They, from what I can tell, had the concrete poured with a "sleeve" left for the sheet of acrylic to slide in and subsequently be siliconed down. I would also have to wager they most likely somehow bolted the acrylic in as well, though I never looked that close.
Just the basic idea and a few of the little bits it's more details and product information I needSounds like you've got it planned out. Keep us updated