Artificial sponge is a good sound absorber, used in acoustic chambers at National Research Council Canada in Ottawa and elsewhere. Maybe a 3-dimensional object made of sponge could be created, to be suspended at the centre of the dome to act as a sound absorber as well as an ornament.
I also heard a few years ago that somebody discovered a spot where the sound resonates in a dome-shaped provincial legislature building in one of the Canadian provinces. The specific point acts as an acoustic focal point.
If a dome building had a paraboloidal top portion which is partially open at the top centre and a paraboloidal bottom portion, like the flying-saucer optical illusion device also known as the "Optic Mirage" sold by Edmund Scientific, then the sound could be focused outside of the dome (cupola) where a sponge sound absorber could be placed, as shown below at the webpage URL for "Life-Size Optical and Acoustical Illusion Device".
The figure also demonstrates that if the inside of the dome is coated with a reflective material, a life-size ghost image would show up at the focal point :-).
Thus, the solution for the acoustic fiasco at the Roy Thomson Hall in Toronto would be to make the interior double-paraboloidal in order to focus the sound at a desired location.
