Read through (little cursorily) the link that you've quoted. Some of the ideas are good, but a little contemporary in direction. For starters, that idea of spraying an insulation is good, but I don't know how much it'll cost. Alternative plan- if you are using a RCC framed method of construction then consider "cavity walls" on the outside. These are walls (in parallel) of single brick thickness with a small air-gap between them. This replicates the "four foot thick wall" that is written about at the beginning, without being nowhere nearly as thick. After all with land prices climbing, one does'nt want to even lose a "postage stamp size" area of floor space. In Rajasthan (back in the good old days) they even flowed water into the cavity in summer! But that would be highly impractical now!
About the 20ft high ceilings, now even that is an impracticality. However one could consider raising the roof on one (or more?) side of a non-airconditioned part of the house to 15+ft or so and incorporating a vent to naturally exhaust hot-air through it . That might even help to give the house an interesting elevation from the outside.
Painting the roof is a good idea, but will need repititive maintenance. Another idea to reflect heat of the roof (a traditional one) was to lay out small bits and pieces of broken glazed tiles into cement on the roof. This was traditionally called "kowdi-kaam" in some parts of India. Though they used broken pieces of crockery initially! The material can be bought by the bagful at rock-bottom prices (since its junk anyway) and after installation is maintenance free. Though one caveat: if your area has decent rainfall, then make sure that the water-proofing on the roof is done well, since you do not want to uproot it the "kowdi" layer to repair it later.
Put most of (or your largest) windows on the North face. Minimise openings on other sides or have shaded awnings on those windows to reduce radiated heat (glare). In the old days, houses had shaded open corridors aka "verandas" running around the house which reduced radiated heat hugely, but may not be practical now. Another feature of old houses was the central open courtyard called the "aangan" It got direct sunlight only for a short part of the day since the structure shaded it most of the time. This feature may be worth re-inventing in a more contemporary and workable form, as it can become a central location where the family and friends can gather together and sit out, even with a barbecue! But its only an idea.
Trees have a great role in shading the property from heat and some-times even as a wind-breaker. Can be used very effectively. According to me, planting a "neem" tree in the sub-continent is a must as it helps to keep insects and pests at bay.
And there is a long held belief that it helps to keep residents healthy, though I can't say much about that.
