I'm luckily in the same boat (or coach) as Nick, with a working interior A/C, but I think I can lend a little guidance here. Since I wasn't at all sure of where the internal support frames are in these vehicles, and since Hall Coach Co is no help what so ever in the way of actual owner's manuals, I bought a very sensitive metal/density detector - kind of a fancy stud finder.
What I found is that there appears to be a total of four (4) frames: one at the front just behind the end of the A/C enclosure bulkhead; one just forward and one just aft of the door opening; and one just forward of where the body begins to curve inward behind the side window. These are approximate, so I would go out and find one of these tools to assist. Another big consideration is that there are DC wires running in the same area for the overhead DC lights. Beware!
A good thing about the Hall roof is that it's very well laminated and shows no sign of oilcanning when you walk on it. You might not need added support at all, but once you cut into the roof you're quite committed. (One of the bragging points that Hall made back in the day was the inside A/C unit because of aerodynamic cleanliness, and you might search around a bit and see if you can find something to go back in the space). I personally wouldn't put a roof AC on just because of the weight and added wind resistance.
As far as awnings go, Nick is right on. Carefree of Colorado makes a nice awning that can be bought with adaptors to handle the curvature of the side of the roof, has no ugly arms attached to the side and stores the fabric in a covered housing (so the fabric won't crap-out so soon). Because there is no wood or metal backing under the fiberglass, all the load will be borne by the fiberglass shell, so you will have to put in a lot of screws. I don't know if they still make it, but in the boat building business we used to use a bedding compound called "Dolfinite" that won't allow crud to grow between hardware and the hull - or body, in this case. Very good stuff that positively keeps out water. Let us know what you find, as this is something I really plan on doing as well.

Matt