A hall like that should not have a dead sound, but is probably too lively right now. Either extreme makes it hard to hear and understand voices.
Your idea of sound absorbing panels is good, but just not the right materials. Sound absorbing material on the ceiling would help a lot and a suspended drop grid ceiling might be a good way to go. So far as I know they are fire resistant.
There are more sophisticated methods of reducing the echo factor, but they would probably cost more. This type ceiling might be cheaper than the panels you described, given the current cost of plywood.
Check into the drop grid ceiling. They are used a lot in offices so I would expect your building dept and fire marshall would approve. To further reduce sound reflection, you could install the drop grid in two or three levels. This would also give it a unique appearance, rather than the somewhat plain single surface.
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