Christoph has a very good suggestion! Should you decide to still paint the APs there a couple of things that you should consider: 1) The warranty will be void, 2) you have to use non-metallic paint to ensure that the RF properties do not change. If you decide to put the AP inside the ceiling, there is the potential for some effect of signals reflecting through the metal hangers. You may want to do a physical survey with a single AP to make sure that the RF propagation is what you would expect. Keep in mind that if you put the AP facing up and resting on the tiles the strongest signal will propagate up away from the floor you are servicing, so you will have to put the AP facing down on the ceiling tiles for the best signal.
Good luck,
Will